How do I make line-in pass through to speakers?

I posted a similar question a while ago, but it's archived now. I would like to use my XM radio through my G4 Power Mac's speakers (no stereo any more).

I can make this work if I run CD Spin Doctor (part of Toast Titanium) and check the box marked "Play input through speakers" in the window where you prepare and start recording. However, as soon as I close that window OR push record and then push stop, the connection between line-in and speaker output is cut off because the sound ceases.

When I was having this issue before, it just magically resolved itself one day but I don't know how/why. The reason that magical fix got messed up is that I just re-partitioned my boot drive and installed Leopard clean (rather than the upgrade I'd done before) to try to eliminate some odd issues like Safari NEVER exiting cleanly if it had had an embedded pdf open. All Apple software updates have been applied, though I haven't reinstalled all my miscellaneous software yet.

So, since I don't know what voodoo got it to work in the first place, I don't know how to replicate the 'fix.' Anyone have any suggestions? There's nothing in the System Preferences that deals with this or has any impact on the issue. Thanks for any help.