On DEC 3000 AXP workstations with DEC-OSF/1 1.3 or higher, there is a STREAMS driver for the built-in baseboard audio (part of the ISDN circuitry that also handles sound), /dev/streams/bba. For more details on the STREAMS driver, please read the manual page for amd79c30(4).
DEC-OSF/1 1.2 does not include this driver, but it is provided with the AudioFile distribution on gatekeeper.dec.com. I haven't had the occasion to test it out, though.
The normal (preferred) way to use DEC BBA audio is to use the AudioFile system, which does for sound what X11 does for Video/Input. Xwpl, however, has its own bba driver, so installing AudioFile is not necessary. I have never tried out Xwpl audio with AudioFile running simultaneously. It's probably a bad idea...
The DEC 3000/300 AXP has a built-in speaker, so no additional hardware is necessary. If you do not have the speaker, you will need to purchase the DEC MMJ adapter (part number 17-03345-01) which plugs into the audio port ang gives you two standard audio jacks. Alternatively, you can also plug in a standard telephone handset with its RJ-11 connector into the audio port.