An easy way to see if your soundcard is working (without relying on third-party apps) is to open the Media preferences panel, turn up the volume, then open a Terminal and press control-G.
If you now hear a beep coming from your external speakers, you have a working sound card. If the beep comes from the internal beeper/speaker, your soundcard is not working with BeOS. Of course, the latter case only means it is currently not working, not that you cannot get it to work by using a 3rd party driver or some such.

Be’s Jon Wätte adds:
To see whether your sound card is supposed to work under version R4 of BeOS, all you have to do is open up a Terminal, and type:

 ls -l /dev/audio/old 

If some name like „awe64“ or „cs4226“ or „sonic_vibes“ or „ymf714“ shows up, your sound card is recognized and should be making sound. If it’s not, you may not have a Beep or Startup sound configured in the Sounds preferences panel: open that panel and choose some sounds and you should be all set.
If you still cannot hear anything, open the Media preference panel, Audio section, and make sure to uncheck all „Mute“ checkboxes in „Output“ and „Mixer“ that may be active.
Of course, the best test is to read the hardware compatibility list found on the Be web site.

Bjoern Wilmsmann (wilmsmann@pwx-webhosting.de) adds:
If your supported soundcard is not working under BeOS, check for resource conflicts in the Devices preferences panel and set it to another IRQ, or put the card in P&P mode (either via jumpers or the BIOS).