Musicians and Singers : Occupational Outlook Handbook : U.S. Bureau of Labor Statistics

Musicians and singers play instruments or sing for live audiences and in recording studios. They perform in a variety of styles, such as classical, jazz, opera, rap, or rock.
Musicians, Singers, and Related Workers

Median hourly wages of wage-and-salary musicians and singers were $21.24 in May 2008.
U$22/hour (rounded up for make it easier to calculate)

It is rare for musicians and singers to have guaranteed employment that exceeds 3 to 6 months.
Lets assume 6 months of 20 days work, 8 hours a day.
8 hours x 20 days x U$22 = U$3.520

So, during six months, in average, a american musician earns U$3.520 per month. So U$21.120 for half-year.

The lowest 10 percent earned less than $7.64
Rounded to U$8

Lets assume, following the considerations of the US government, that the rest of the year the musicians in unemployed, that instead, the musician receive the lowest possible, but receives in the same regular manner.

Lets assume 6 months of 20 days work, 8 hours a day.
8 hours x 20 days x U$8 = U$1.280

During the other six months, in average, he will earn U$1.280 per month. So, U$7.680 for the other half year.

We have an average income of U$28.800 / year for the majority of the musicians in US. Rounding it to U$29.000

Now about the related workers described there:

Median annual wages of salaried music directors and composers were $41,270 in May 2008.
I'll round to U$41.000

Considering that US, acording to the text, have 240.000 professionals musicians, out of those, aprox. 186.000 are musicians/singers and 54.000 are directors/composers (probably producers are here too).We have that 77,5% of those are musicians earning U$29.000 and 22,5% earning U$41.000 a year.

That leads to a new percentual average of: U$31.700 / year

Multiplying U$31.700 x 240.000 (professional #) we have

U$ 7.608.000 (rounded to U$7.6 billion dollars)