Is it possible to be listening to, say, a radio broadcast, and to route that straight to GB in real time?
not with GB itself, no, you'd need some third party software like SoundFlower to route the audio stream to GB
Or do you have to save it somehow and drag the saved file into GB?
that would be the better/easier way, something like Audio Hijack, or WireTap (among other software) can record audio from other apps, and save to disk
Thanks, HangTime. I can't see any of doing it just with the software and hardware of a Mac device, however... I just did an experiment (should have done it earlier, I guess!). Using an iMac and a Presonus Audiobox USB (which is basically my recording setup), I took a stereo send out of the headphone jack of the iMac and set the iMac's output to "Headphone". This send turns into two 1/4 inch (guitar-type) plugs, which I put into L and R of the AudioBox. I opened up a GB audio track and set the input to "Stereo/Audiobox 1/2", and set the output also to the Audiobox. Then hit REC on GB and PLAY on a YouTube clip--"Dancing Queen" by ABBA (yeah, I know...), and it records in real time. So, the chain is: Youtube clip, played through headphone jack, ANALOGUE INTO Audiobox, USB INTO Garageband Track, with audio out through the Audiobox. You have to play around a little with the volume going in and the record levels on the GB track, but I got it sounding good within about 5 minutes. I thought it would squeal in some extreme feedback loop, but that didn't happen. The CD I burned was only slightly low in volume, but I did get the benefit of boosting the bass end in GB. Just saying that it can be done after all. Obviously, you need some other device to physically send the signal, or (as HangTime mentioned) some other software to digitally route the signal. The digital option involves the least amount of plugs and devices.