Not silence, then people will think they got cut off.

Ambient and soothing sounds. Like waves on a beach or something.

Probably lofi or classical music but silence also works well.

I like music. You can put the phone down and still hear the constant "still on hold" sound, when it stops you know it's being answered.

Given to me it's a tool for that purpose, the actual music doesn't really matter as long as it's constant and lound enough. Probably best not to be any music I like, I'm never waiting on hold for good reasons so best not to associate songs I like with a bad experience.