*Update*
Zoom have thankfully now added an option to the mobile app to enable “original sound” on those devices too. To do it, you need to turn on the option in Settings BEFORE you start or join the meeting, then go to the “...” menu and you’ll now see an Enable Original Sound” option. More details here: • Getting Good Audio For Music In Zoom On Ph...
If you are currently using Zoom to play music online, there are a couple of things you'll want to do to get the best audio quality you can. In this video I show you how to do those things!
tl;dw:
Don't use a phone or tablet, connect with a laptop to have access to the settings you need to set
Find Audio Settings... in the menu that pops up next to the Mute microphone icon
Turn OFF "Automatically adjust microphone volume"
Go to "Advanced"
Turn ON "Show in-meeting option to "Enable Original Sound" from microphone
Close settings
Make sure there's a button in the top left of the screen that says "Turn off Original Sound" - if it says "Turn on Original Sound" click on it!
For extra goodness, try using an external audio interface to hook up a good microphone or two!