From: Steve Gardham - PM
Musicians' session.
usually equals almost everybody in the room thrashing away on an instrument oblivious to anything else but the tune.

Singers' session.
Individual singers performing, everybody else listening.

A big difference I'd say.

Discussing 'manners', this is one that annoys me.
Can we stop discussing musicians/singers? The term 'musicians' encompasses both.
I think (hope) that the distinction you intended to make was instrumentalists(or players)/singers.

Cheers
Nigel