I've done a little recording of audio from external sources onto my Amiga. I used a program called AHIRecord which I obtained from Aminet. It's a very very simple program, but it does what it says on the tin and that's all you want really isn't it?
To use the program you plug all your equipment in, play the source, then set your recording level up (there's a bargraph to give you a visual representation) so as to avoid distortion and then that's it - just press "record" when you're ready to go.
There might be other better alternatives if you're after something which provides more control, but I found AHIRecord to be a nice "simple but effective" program.
Brian