allocate_voice - Allocates a sound card voice for a sample. Allegro game
int allocate_voice(const SAMPLE *spl);
Allocates a sound card voice and prepares it for playing the specified sample,
setting up sensible default parameters (maximum volume, centre pan, no change
of pitch, no looping). When you are finished with the voice you must free it
by calling deallocate_voice() or release_voice(). Allegro can manage up to 256
simultaneous voices, but that limit may be lower due to hardware reasons.
Returns the voice number, or -1 if no voices are available.