This is hell of a lot easier today.

If someone is maybe still requiring to convert m4a files to wav format, there are tons of free online converters. I would honestly prefer using some of the premium tools that you need to buy or at least pay subscription. But, if you think that is unnecessary and still want to do it for free, my recommendation is to watch out on what websites you visit and what converters you use. I personally use Converter365, find it
here. Pretty simple. I had no problems converting larger files.