I would like to know whether the number of musicians in the US is
increasing or decreasing and please include a credible source.

Dear infopros,

After extensive research, I came to the following conclusions:

- The number of musicians in the USA has significantly increased
between 1990 and 2001.

- But future trends are impossible to derive since older data shows
that the total number of musicians in the USA is very much subject to
fluctuations.

My results in detail:

The most reliable source available for the number of musicians is the
US Census. Although census classification does not distinguish between
musicians and composers, the data should give an acceptable impression
of the numbers we have to deal with. Also, there is no more detailed
source at all.

These are the total numbers for the occupation category "musicians and
composers" as published by the US Census Bureau:

1910: 139,310

1920: 130,265

1930: 165,182

1983: 155,000

1990: 138,330

2001: 161,000

This clearly shows that the number of musicians and composers has
increased by ca. 22,670 between 1990 and 2001; but one has to keep in
mind that this number has already been 155,000 in 1983, so there was
an interim decrease.
Also, the earlier data demonstrates that there is relevant fluctuation
in the total number. This fluctuation is even more grave when you
think of the fact that the total population number of the USA was
smaller in the past. So the percentage of musicians and composers
among the inhabitants of the USA is rather decreasing in the long term
point of view: The USA had 122.9 million inhabitants in 1930, while
there are 281,4 million US-Americans in 2001. So the percentage of
musicians has shrunk dramatically in those 71 years.

Sources:

DoHistory: U.S. Census of Population, Occupations, Vol. 5
http://www.dohistory.org/archive/doc099/099_p20_txt.html

U.S. Census Bureau: Radio Zone - John Philip Sousa
http://www.census.gov/pubinfo/www/radio/pa1106.htm

U.S. Census Bureau: Statistical Abstract of the USA, Section 12 -
Labor Force, Employment and Earnings (Acrobat Reader file!)
http://www.census.gov/prod/2002pubs/01statab/labor.pdf

RAND: Artists - Creators and Performers (Acrobat Reader file!)
http://www.rand.org/publications/MR/MR1367/MR1367.ch5.pdf

Search terms used:
"us census" musicians
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22us+census%22+musicians&meta=
"us census" "musicians and composers"
://www.google.de/search?q=%22us+census%22+%22musicians+and+composers%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=0&sa=N
1990 census musicians figures
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=1990+census+musicians+figures&meta=

Hope this answers your question!
Regards,
Scriptor

Good job.  Gets me to the next level.