There's a post here showing how to route all sound through the earpiece
http://talk.maemo.org/showpost.php?p=800577&postcount=2

Having had no sound through the earpiece for phone calls at all yesterday, I tried running the script and all sounds were indeed routed through the earpiece.

Then rang my landline on skype and audio worked. Rebooted the device and audio was still working.

This morning, audio was no longer coming through the earpiece on incoming cellular calls. Ran the amixer commands again and still no audio, so I was starting to suspect a hardware fault.

Switched profiles to silent and back to quickly reset the audio settings and then audio came back through the earpiece on incoming cellular calls.

There's a general issue logged in bugzilla somewhere about dropping audio on phone calls which I'll try and find time to update today with this info.