Muffled audio input AND output on Mac laptop

Hello - suddenly one day recently, my output audio (what others heard) became very muffled.  They could hear me only if I shouted.  Then today I tried headphones in a meeting, thinking maybe that could fix the speaking issue.  It did not.  However, upon removing the headphones, now the speaker output is very low.  I can now barely hear the other participants.  I have an older Mac Pro laptop, running High Sierra.  It was all working fine and only recently has this audio issue.  And only with zoom.  I have updated zoom, and checked all audio settings, both on the mac and on the zoom app.  I reset the NRAM or whatever at the Apple store.  Any suggestions?   Thanks!!  This is making zoom unusable.