Plays a sample.
Triggers a sample at the specified volume, pan position, and frequency.
The parameters `vol' and `pan' range from 0 (min/left) to 255 (max/right).
Frequency is relative rather than absolute: 1000 represents the frequency
that the sample was recorded at, 2000 is twice this, etc. If `loop' is not
zero, the sample will repeat until you call stop_sample(), and can be
manipulated while it is playing by calling adjust_sample(). Example:
/* Scream from the left speaker, twice the freq. */
int sound = play_sample(scream, 255, 0, 2000, 0);
Returns the voice number that was allocated for the sample or negative if
no voices were available.