the microphone. Buy a quality one. Given what you're recording (viola and piano) I'd suggest a pair of what are known as "pencil condensers"; to wit, small diaphragm condenser mics. Note that condenser mics almost universally need phantom power (which is a 48V tension between the hot and neutral plugs to deliver electricity to the mic), which also means you're going to need something that you can plug it into that provides that. If you want to go really nice, something like a matched pair of Rode NT5s is a really good choice: https://www.amazon.com/Micr%C3%B3fonos-condensadores-NT5-MP-haciendo/dp/B0002PZ01C/ If that's too much money there's a pair of Rodes that cost a couple of hundred bucks: https://www.amazon.com/Rode-Compact-Condenser-Microphone-Matched/dp/B00GJW0JGI

The next thing you'll want is a good ADC. You'll want one with at least two inputs (because in order to effectively mic a piano you need two mics). I've been using focusrite and behringer gear (the recording up there is done with my X32 because INPUTS) but you don't have to get a big-ass multichannel mixer for what you're talking about. Something like a focusrite scarlett 6i6 might be a good choice; they have one of the best noise floors in the industry, but there are cheaper alternatives out there. Don't get an m-audio; they have a history of requiring proprietary drivers (when the USB sound spec is plenty fine) and abandoning driver support for older hardware where that hardware realistically still has five to ten years of life left.

Again, and this is really important, make sure your interface supports providing phantom power to the mics!

You can record at up to 96KHz sample rate with the focusrite, but realistically you won't need that; I'd suggest half that sample rate as that's become the de facto standard for quality audio on the internet (and that makes sense given that it's ~10% higher than CDs) so realistically anything you put up for folks to download and listen to will get downsampled to that rate anyway. 96KHz is for people recording albums at Sony or something, and even then the resultant product is getting downsampled to 48KHz before getting sent to spotify etc.

All of those are USB 2 devices; turns out usb2 gives plenty of bandwidth for high quality audio. The next issue is software. For the recordings you're talking about (Aidan playing at home), audacity is plenty fine. If you're running windows and want to be able to do more stuff, the free tier of ProTools might be the way to go. If you're in linux land, you could look at Ardour. It's a PWYC software. However for the quick and dirty audacity will totally work for you, and even has decent effects you can apply to the recording if you feel so inclined (compressors in particular are Freakin' Handy).

Proper micing of the instruments is also very important... so to start with you can add a couple more items to that list; in particular I'm thinking of boom mic stands. You should be able to get a pair of those for ~130 bucks (and I'm talking Canada prices here; should be even less for you). For the viola, you want the mic pointing app. at where the neck meets the body (and NOT at one of the f-holes; you can get some fun resonance/proximity effects that will ruin the overall experience of the performance) at a distance of about 4-6". For the piano divide it into quarters, and use the stands to place the mics above the piano at the one and three quarter marks side-to-side and pointing directly at the strings. Your goal here is to get two equilateral triangles with the two mics at the apex pointing down. Depending on the piano you may or may not want to lift up the top; that's something to play around with when you're actually setting up the gear to find out what works best with your equipment and room. If it's an electric piano of some kind just plug it directly into the interface.

Hope this helps point you in the right direction; if you have any questions about any of that feel free to hit me up!

that that's gonna work; I mean, I could see it working for a while maybe, but I'd think sustained use would result in xruns or dropouts. Given how much it should cost to get a new one, I'd really suggest getting a USB one instead.

that is that the next sample is arriving while the machine's still processing the previous one, and it gets dropped. This results in two hard pops at each end of the sample as the wave slams to zero for the length of the sample and then slams back up to whichever value the next sample has. This usually doesn't only happen once so you end up hearing a series of hard clicks/crackles.

If you are experiencing that you can try a few things. First would be increasing the size of the input buffer. First step is to bloat it all the way out; if that doesn't fix it you're very likely to be dealing with a fault in the unit. Yes that'll mean serious input to output lag, but afterwards listening to the recorded track will tell you if that's an issue; if the crackles disappear that means you'll be wanting to look at the computer instead of the unit. If that doesn't help, maybe open it up and see if there's dust in there causing the chips to overheat?

Crackling and xruns are a large part of the reason why I'm highly highly sceptical about getting it to work with an adaptor....