-- recording directly to .wav using an emulator
If you have your .sav and .gb on a computer, it may be possible to record your song without the audio leaving the digital realm. Check out Gameboy Emulation and see if there are any programs which have .wav output functionality for your operating system. If not, you can use a Screen Recorder to Record a sound file from your Emulator.
— recording using a microphone
In live situations, your only option may be to use a microphone to record your music. However, microphones “color” the sound of everything they record. This is true even of expensive/high-end microphones, but don't be scared of coloration, it can add character.
I had been recording using my laptop's built-in microphone and had just assumed that the Gameboy sound chip only used 3 diminishing harmonics. My first recording using the patch cable and adapters revealed 7 harmonics in one instrument. All of them were nearly clipping and at the same volume. That's something to impress the nonbelievers with!
— recording analog signals over patch cables
Another common method of recording tracks is straight out of the 1/8" audio jack on your gameboy. Each model of gameboy sounds slightly different, it may be fun to try all the options out. Just remember: what you have works =)
Depending on the audio-jack-input type of your soundcard, you will have to choose the right cable to to get the analog audio signal safely out of your gameboy's 1/8" jack.
Besides finding the proper cable, you can also string something together using adapters. However, Less connections mean less interference, so the fewer adapters the better.
Audiophiles claim they can hear the difference between regular cables and those with the 24K Gold covering on the tips; Radio Shack makes cables with and without this covering. Interference can be dramatically reduced, for ultra-clean spaces between the sounds.
check out the page http://www.guitarnuts.com/technical/cords/index.php on his description of audio cables and what to choose for what application. he talks about the uses of gold plating with a guitar cable but in this instance, it would be no different then using a gameboy, they're both instruments. because of the frequent use of the cable the gold will eventualy just wear off or chip and become just regular old connections again. the only time gold plated plugs seems usefull is in connections that get plugged in and stay that way for long periods of time, i.e. speaker cables in a p.a. system. some have also entertained the idea, that spraying the connections with deoxit before every use plays on the sound quality, which i could see being more likely, but have no proof. the things he highlights in his articule to look for are quality in construction. thats the main thing. oxygen free cableing only seems usefull in speaker connections as well. so read up on all that before dropping big bucks on a cable that may not actualy do anything other then beat up your wallet. -sameal
— some products you may need for recording
1/4" to 1/8" adapter: 1/8" cable: 1/4" to 1/8" cable:
Here is a better cable: http://www.musiciansfriend.com/accessories/hosa-stereo-y-cable-mini-male--2-1-4-mono-males
And for those with DJ mixers: http://www.musiciansfriend.com/accessories/hosa-stereo-1-4--2-rca