No sound coming from speaker

I've dragged my old S8 out of retirement to fill in while my current phone is being repaired. Trouble is, I'm not getting any sound from the speaker. Oddly, I can talk on the phone OK but it doesn't ring nor vibrate, so I have no idea if a call is coming in.

The lack of sound applies to app, including YouTube, That doesn't bother me too much, but missing calls and other notifications does. I can hear sound through headphones but I refuse to wear them continuously.

I've tried another speaker module (this is a bit of a frankenphone, repaired from a couple different S8s) but no luck either. This phone's speaker was working before I replaced the screen so I suspect that something isn't plugged in property, but after going through the various guides, I can't see what it could be. The guides just show the speaker module being put in place and screwed down.

Any ideas?