Open music.cfg located in data>sounds>music folder and add a ; before every file you want disabled. Also you will need an "empty" file to replace originals in that cfg file (you can find one in GWX

.
So - it should look like this:
[VICTORY]
Tracks=1
Track1=empty.ogg
;Track1=Happy_Times_1.ogg
;Track2=Happy_Times_2.ogg
;Track3=Happy_Times_3.ogg
You can also select which track the game should play and which not by adding ; before unwanted track(s):
[TENSION]
Tracks=3
;Track1=Empty.ogg
;Track1=Tension_1_Mix.ogg
Track1=Tension_2_Mix.ogg
Track2=Tension_3_Mix.ogg
;Track4=Tension_4_Mix.ogg
Track3=Tension_5_Master.ogg
Also its important to specify number of tracks in each section: if you had 6 tracks but disabled 3 then you must change "Tracks=6" to "Tracks=3".
I could send you my "empty.ogg" (from GWX) but maybe its copyrighted or something...?