I don't know about getting the sounds off the biosonix but if their underwater speaker isn't hard wired to the box, you might be able to plug in a recording line in to the biosonix box instead of the speaker and record the sounds to your computer. I don't know about all that because I haven't seen the unit in person to look at how it's all hooked up.

Getting you own sounds would be very difficult. You'd need a hydrophone and some great conditions to get audio of fish actually feeding. I don't know how the Biosonix people got their sounds but I have to think it was in a farm pond or some environment where you could get close enough to the fish to record them on a nice calm day where other noises wouldn't screw up the recordings.

I don't know much about recording underwater sounds but you can buy a hydrophone, which is an underwater microphone, for $200-$300. I assume they have a standard line out that you could plug into a camcorder, laptop, or into an mp3 player directly. Then just use whatever device you have it hooked up to to record the sounds to wav or mp3 files. Copy from there to your ipod and you're good to go.

I'm not about to go do it, I'm just pointing out that the device doesn't have to be so big.