detect_midi_driver man page
detect_midi_driver — Detects whether the specified MIDI sound device is available. Allegro game programming library.
int detect_midi_driver(int driver_id);
Detects whether the specified MIDI sound device is available. This function must be called _before_ install_sound().
Returns the maximum number of voices that the driver can provide, or zero if the hardware is not present.
There are two special-case return values that you should watch out for: if this function returns -1 it is a note-stealing driver (eg. DIGMID) that shares voices with the current digital sound driver, and if it returns 0xFFFF it is an external device like an MPU-401 where there is no way to determine how many voices are available.
install_sound(3), reserve_voices(3), set_volume_per_voice(3).