Can't get sound working through line-in correctly

[dpb]

In short: I can record and play it back and hear it while playing back, but I can't hear it while it's happening (monitor). I've gotten it to work before by messing with mute levels in alsamixer and gnome-alsamixer, but now it's not working.

Am I missing something? Seems like I'm so close, don't want to give up now!

Thanks!