I like tools that take a few parameters and then procedurally generate something based on your input. Creating a music loop is a perfect place to use a procedural tool. The tool I’ve found useful for generating some simple music loops so far is called Seaquence. Basically it gives you a creature looking thing that swims around and embodies the current settings of your procedural music loop. In the tool there are what appear to be 5 different wavelengths you can choose from for your loop. Each option is a different color. You are then able to modulate the wavelength using some points along it’s length. The frequency of the waves in the wavelength are controlled by a plus and minus which allow you to increase or decrease the number of waves in the wavelength. Finally there is a grid that the wave runs down and plays a sound if it hits a point on the grid. You add points to the grid and then it basically plays a music loop for your based on your settings. It sounds complicated but basically you just click around until it sounds to your liking and then you can record it (with Windows Sound Recorder or another tool) for use in your Android, IOS, OSX, and Windows apps. Lastly you can add multiple creatures to the same “sea quence” and they swim around each other. The closer they are to the center of the screen the louder the music from that creature sounds. You can even share a URL to your music creation.
If a simple music loop like this doesn’t fit with what you are looking for with your app or game you can go with a more traditional route. There is a site called Shockwave Sound and they provide all kinds of professional music licensing around $10 to $40 per music track. They have all kinds of music here. Everything from classical music, to hard hitting epic soundtracks, to corporate presentation music, to guitar pieces, to Hawaiian music, to techno, and much much more. The music is created by both hobbyists and professionals.There are quite a few gems in their collection but you will probably have to preview quite a few music pieces until you find one that fits with what you want. They even license out sound effect packs. Their licenses are pretty decent and they allow you to use the music in things like games and apps. I have licensed quite a few pieces of music from them over the years. They provide full previews of the music (with a watermark of coarse) so you know exactly what you will be getting beforehand.