GIMP - an old favorite of mine
OpenCanvas
Audacity - good program for sound.
I see. If you can't find someone to agree to work that way (Scrum), and don't want to pay by the hour (which has its positives and negatives), you might also be able to negotiate based on each overall asset.
Even asset prices can differ, though. While a tree sprite might cost $10 or less, a fully animated 2D main character could reach $30-$50+. Factor in a bit more than that if we're talking 3D, especially if the animations that are needed are complex.
I feel it honestly depends a lot on experience of the candidate(s) chosen. Need someone with a college degree? That may cost more.
You might be able to get a startup for little more than $10-$15 an hour. But to hire a seasoned artist/musician with industry experience, could very well run $35-$50+ an hour.
If you can only offer a lower price... it may also provide incentive if they can do the work from the comfort of their own home.
Lastly, for small jobs, you might also look at something like Fiverr, and browse the really high-rated artists/musicians with impressive portfolios.
Just some food for thought / tips.
1. Loopable tracks. Plenty of game songs out there take one on an emotional journey. But some just aren't "game ready" and fit for certain games.
2. Number of tracks, since some games require shorter music tracks. For example, if a level is 2 minutes long, it may be a waste to use an 8 minute music track.
3. I'd rather pay for a higher quality.
4. Yes. For sure.
5. There are a lot of places to look. Itch.io is just one of them.