Consolidate I_SetDigMusicVolume and I_SetMIDIMusicVolume into one method

* In s_sound, they are merged to one method as well, but there are still two separate digvolume and seqvolume variables
* Simplified Dig/MidiMusicDisabled in s_sound
* Method reordering

(cherry picked from commit 701cc5a7dd1dfead87a42ec7558c9fa6a1deb193)