For the main topic: I've tried a lot of different SAHM jobs, from Lionbridge (same type as Leapforce), to translating and proofreading, to photography. DH claims he has made a bit of money buying old musical instruments, fixing them, and selling them. I think he has just got a nice collection of instruments for a relatively low price.
Well, I haven't really looked deeply into music, at least not deep enough to study the competition to advise you the best place to work with. I think first of all you could go here, give it a look, read the articles, do some searches related to the kind of thing you do, look up keywords, and then visit the forums. People there are very receptive and quick to give advice. They can easily give you tips on how to work with our audio bank, or to point you to external forums that might open up new possibilities.
The thing with royalty-free stuff is that you can be very authoral and creative, but you're not selling your music for yourself, your fans, etc. You're selling it to become someone's tool. You'll see it in Youtube video tutorials, website backgrounds (eeek), advertising (some people are experts in making jingles that the client just need to add a set amount of words to), maybe in an online game, or a corporate presentation. I think mostly YouTube, Vimeo and TV ads, though. :) Oh, yes, sometimes radio podcasts as well. :)
What we usually say to musicians is: if you have a studio hour hired, you can use 55 minutes to record your songs and then 5 minutes to record a short track or two to sell RF. It could even be just the sounds of the band dismounting after recording, some people laughing, or a short generic conversation. (I get lots of stares of the "but each single minute is sooo precious!" kind) - Most people just use they own devices at home, though.
Edit: Oh, yes, and I recommend audio because there's still so little content available for customers, in all of the stock market. If you're really interested I could see if I can find the contact of someone working in the audio section to ask more pointed questions about what's selling well, trends and tips.
This is an old post, but if there are other people reading reading through, I'd like to get in my two cents. I agree that stock photo, photage and music is a great side gig/passive income stream, but I do not agree with farmstache on a few things:
-$1/piece/month is for high quality stuff. You could just as easily make $.1/piece/month. And the rate keeps dropping as the competition increases.
-Stay clear of iStock. They keep a much larger share of the money than all the other stock agencies, you never know when they are going to change the rules on you, and several times they have given away stuff for free without asking the artists. Payouts get delayed for months, and you can get kicked out if you voice a protest about anything. My blog hasn't been updated for years, but the (affiliate) links to the sites still work, and the statistics over best selling and least selling sites are still true:
http://gajasworld.blogspot.no/ . (Delete the last part of the links, or go directly to the sites by searching for the names if you prefer to avoid affiliate links.) The best for music, in my opinion, is Pond5.
-Be very careful when using models. One of the guys at microstockgroup.com is getting sued now by a model that found her pics used for porn. Not something you want to experience with a friend or aunt. I prefer shooting rocks. They don't argue.
I've had fun building my zazzle stores. It doesn't really make a lot of money, but ones the job is done, the income is passive. Also, I only make products with bad puns, so at least I get a laugh out of it.