Kevin, to investigate this further, you'll want to step into detect_digi_driver or install_sound and see what happens.
You can specify one of 5 audio drivers on *NIX using the following constants :
DIGI_OSS - Open Sound System
DIGI_ESD - Enlightened Sound Daemon
DIGI_ARTS - aRts (Analog Real-Time Synthesizer)
DIGI_ALSA - ALSA sound driver
DIGI_JACK - JACK sound driver
Try calling detect_digi_driver before install_sound on each of them.
I can try it on a Ubuntu 18.04 VM I have setup, but that's no guarantee it will or won't work with regular hardware.
install_sound will give you back an error if a problem happens :
int install_sound(int digi, int midi, const char *cfg_path);
Initialises the sound module. You should normally pass DIGI_AUTODETECT and MIDI_AUTODETECT as the driver parameters to this function, in which case Allegro will read hardware settings from the current configuration file. This allows the user to select different values with the setup utility: see the config section for details. Alternatively, see the platform specific documentation for a list of the available drivers. The cfg_path parameter is only present for compatibility with previous versions of Allegro, and has no effect on anything.
Return value: Returns zero if the sound is successfully installed, and -1 on failure. If it fails it will store a description of the problem in allegro_error.
I can get a DIGI driver, OSS if I ask for it, and the play_sample function returns a valid voice id, but there is no sound.
I got a trumpet wav file to play, but failed with another french horn sample wav.