This post should serve as a little guide to the resources available on-line on the topic of audio levels in different media. It has been compiled due to the big frequency of questions on the topic, and thanks to the big amount of answers in this forum!
Since audio in media is an ever-changing field, this post will be updated as I stumble upon new and interesting infos or links. If you have insight into data that you think should be included or corrected, please PM me, or post it here.
For further, specific questions on mixing levels, you can post in this thread, or start a new one. 
DISCLAIMER: if you're coming from the MUSIC MIXING/MASTERING BACKGROUND, read this thread first (posts 1-5): GENERAL Average DB settings?
Movie theater
There are no guidelines in terms of average loudness, peak or any other level measurement. You achieve proper levels by properly calibrating your listening environment, so that it resembles the environment of the theater.
To calibrate your room, read this:
DUC: Room Calibration for Film and TV Post
(or,
in a nutshell) (or,
a tutorial video) (or,
as a last resort, if you can't get an SPL meter)
Then mix by ear. "If it sounds good, it is good" - JoeMeek.
Here's a useful discussion:
https://gearspace.com/board/post-pro...st-levels.html
However, there is a maximum loudness level for
theatrical trailers and commercials which is measured with the
Dolby Model 737 Soundtrack Loudness Meter.
Trailer loudness should not exceed 85 dB Leq(m), as regulated by
TASA.
Commercial loudness should not exceed 82 dB Leq(m), as regulated by
SAWA.
DVD
Here, same rules apply as with the theatrical mix, except that the monitoring is different (near-field, no X-curve), the room is smaller, it is
calibrated lower, AND there is the
dialnorm parameter if your sound is AC3 encoded.
Read about dialnorm here:
https://gearspace.com/board/1112798-post7.html (section about Dialogue Level)
and here:
Home Theater Hi-Fi: Dialogue Normalization
You have to determine your target dialnorm BEFORE you start mixing, so you can adjust your listening level accordingly. Most DVD's are mixed for dialnorm -27dB (because that setting is the most compatible with the theatrical mix), but some use the full dynamic range (-31dB).
TV (everything BUT commercials)
Every broadcaster has its own specs. You have to get the specs of your target TV channel.
Detailed Specs
They can be very detailed, like the
Discovery specs or the
PBS specs (section 3). They will tell you exactly what is your max peak level, average dialogue level, average overall level, what measurement instrument is to be used etc. Meter that the US networks usually specify is
Dolby LM100 (*this sentence needs an update, as it seems a lot of US mixers are using different software meters these days*)
As of late 2010, European broadcasters are beginning to encourage the
EBU R 128 recommendation, which stands a chance of becoming a de-facto standard for Europe. France and some other countries have started using R 128 since Jan 1st 2012.
Check out this great document by
Florian Camerer of ORF and
PLOUD:
On the way to LOUDNESS nirvana
Here are two threads about mixing against LM100:
Mixing with the Dolby LM100
Anyone have experience mixing while adhering to specs monitored by the Dolby LM 100?
This is great! A post by Mark Edmondson, Audio Post Production Supervisor at Discovery:
Dolby LM100 and Discovery deliverables - Digi User Conference
Basic Specs
The other extreme is on the minimalistic side, like the
RTL or
BBC specs which give you only the
maximum peak level, and the
reference level. This is what it's like in most of the Europe, except for the countries where R 128 was adopted.
- REFERENCE LEVEL - it is used for equipment alignment, and doesn't have a direct relation to actual mixing levels.
In EBU countries it is -18dBFS and corresponds to electrical level of 0dBu (per
EBU R68).
In SMPTE countries it is -20dBFS and corresponds to electrical level of +4dBu (per SMPTE RP155).
Sometimes refered to as: Zero level, Line-up level, 0VU.
Broadcast Audio Operating Levels for Sound Engineers
Reference Levels on Common Metering Scales
The Ins and Outs Of (Sound on Sound)
- MAXIMUM PEAK LEVEL - this is where you set your brickwall limiter on the master buss, or otherwise not go over it (although in some of the specs, short peaks of 3 to 5 dB over this value are allowed - go figure!). Usually -9dBFS or -10dBFS in the continental Europe, PPM6 in the UK and Ireland.
What can make the confusion here is that the
average dialogue level is not exactly specified.
In a perfect world, you would
calibrate your listening environment to the ITU-R BS.775-1 standard (-20dBFS pink @79dB SPL/C/slow), [or
EBU 3276 and
EBU 3276-S if you are in Europe] and then mix by ear. In that case you would get average dialogue levels at around -27dBFS RMS.
However, this way, your mix could turn out too quiet, as there's a loudness war in broadcasting, probably in part due to the loudness of commercials and the loudness war in music.
Average dialogue loudness that works for me (dramatic program, regional stations in the Balkan peninsula) is -23dBFS RMS. To achieve that, I calibrate my monitoring to 75dB, and thus reduce the headroom by 4dB when compared to the ITU's 79dB reference.
However, your best bet is to talk to someone who regularly delivers for the given broadcaster or in a given market, and ask him about his average dialogue level, or how his listening is calibrated. Chances are someone at this forum will be able to help, too.
Further Reading
More about broadcast delivery specs:
https://gearspace.com/board/1181448-post23.html
A great intro to broadcast audio:
Audio for Digital Television
A compilation of links about Loudness in Broadcast:
Loudness Links repository
A seminar on the topic and more:
CAS Seminars - 'What Happened to My Mix?' - The Work Flow From Production Through Post Production - Cinema Audio Society
Dialnorm was to be implemented in broadcast too (as Dolby imagined), but it isn't, so far (and likely won't be):
DTV Audio: Understanding Dialnorm
Food for thought on setting up variable monitoring level:
Bob Katz - Level Practices
TV commercials
Again, you have to get the specs of your target TV channel, but you will most likely only use the max peak value they provide. Below that, you can compress as much as you wish - it's a loudness war, similar to the one in popular music production. This is changing fast though, and now some US broadcasters and some European countries have proper loudness specs - here's a useful thread:
Loudness specs regarding TV commercials In EU countries
These are some of the legal efforts on loudness regulation of commercials:
US:
H.R. 6209: Commercial Advertisement Loudness Mitigation Act (GovTrack.us)
UK:
UK commercials for TV - perceived loudness issue - Digi User Conference
Congress getting involved in television advertising levels.
Internet / Youtube / WWW
No rules, but most people seem to be allowing the peaks up to -1 or -2 dBFS or dBTP, while the average loudnes is between -15 and -18 LUFS or LKFS. Read all about it here:
Loudness levels for internet teaser-trailer
Radio
Nothing much to say here:
Radio Broadcast Levels
A BBC technical specification:
BBC Radio Resources // Programme Delivery // Glossary
Less is more (straight from the horse's mouth) - Bob Orban and Frank Foti talk about what goes on with your mix in the radio station:
Radio Ready: The Truth
Games
Absence of standards:
Video Game Reference Level/Standards
THX: Establishing a Reference Playback Level for Video Games
A thread at SDO with some advice and some official information from Sony and Microsoft:
Niveau Sonore en jeux vidéo :: Sound Designers.Org (
Babelfish English translation)
(Note: the Xbox360 document is in English)