The problem with Zoom is that it was not made for singing but conferencing and it will interpret long held notes and instruments as "background noise" and try to eliminate them. Enabling original sound cancels this and improves the musical sound.

It is also recommended to unclick "auto adjust volume" in the audio settings.

Have your own sound turned up to about 90/95%

It is also recommended to go into the "advanced" audio settings:-

make sure "enable original sound" is "on"

but "disable" both the settings relating to "persistent background noise"
but leave "echo cancellation" enabled.

This will ensure that you get a reasonable sound, although problems with internet connection can also affect things, but this is really out of your conttrol.