I've seen older posts on this, was hoping that by now Ring has come up with a solution:

The Base Alarm is not loud enough to hear upstairs or downstairs. I had a Dome Siren, it wasn't very good but is now dead. It appears these are no longer made which is not surprising.

It appears the only options are buying a Ring outdoor siren(for inside) or buying an Echo Dot and programming that to Ring ? Does anyone have a different solution ?

This seems like a big fail on Ring to not account for this.