I guess try the obvious, push down on the black knob and see what the volume setting is at. I usually run around 7-10.
Also each channel/talkgroup have the ability to lower the volume up to -3 from the volume you set, check that.
Uniden imho had a bad run of 396xt's where the speaker would go put completely. Worked on several dozen 396XT's, never seen one that had low volume, it worked or it didnt.
I have seen first hand some half dozen 396xt's just in my area including one of mine where either the speaker is shot or circuitry that drives the speaker needed repair.