I have used Linux for many years and MxLinux for several years. I currently am running Mx-19.2 with all updates as of today installed. My hardware is an Intel NUC with either headphones plugged in or Clarium Bluetooth ear buds. I am using the default audio setup which is Alsa, Device=default (have also tried HDA Intel PCH), Channel = Master (have also tried PCM). Until the other day, everything was fine. FWIW, I have always had two speaker icons show up on my launch bar. I usually just close one (quit), but it has never made any difference. Either one seems to work. Now, suddenly, I get sound normally from Firefox and from VLC, but NOT from Clementine or Pithos. To make things even stranger, Pandora works if I connect via Firefox, but Pithos does not. When I open the mixer, it shows a listing for Clementine (and the name of the song currently playing) or Pithos, but no sign of audio coming from them whereas Firefox or VLC shows the pulsating volume as expected. Given that one internet source works and the other does not and one local source works and the other does not, I have no idea where to start trouble shooting!
This is one of the weirdest problems I've had.
Any / all help or suggestions appreciated.

Have you tried the MX Linux forums? https://forum.mxlinux.org/ I'm not familiar with MX, so I would start there for specific help.