Well that's weird. I don't see why it would go bad from doing the usual screen replacement.
I'd start off by ruling out software first.
Restore your iPhone via iTunes (Make sure to do a iTunes backup of the phone if you need to keep any user data off it) then restore from a iTunes backup and test the loudspeaker / ringer.
I recommend trying without restoring the backup then trying after (Sometimes the phone settings stored in backup can do weird things with sound / mic not working).