(from New York Film Academy)
As every voice is different, there are no hard and fast rules as to the perfect song selection but there are some handy rules of thumb in order to get you on the right path.

Naturally, you’ll want to avoid anything with a vocal track since you won’t want the VO competing with the singer when it comes to holding the audience’s attention. Similarly, music with high-end instrumentation (such as clarinets or violas) can clash with the vocal frequencies, so aim for something with mid to low level tones.
Don’t attempt to fluctuate the sound levels of the voice or music track – for instance, upping the voice during a loud part of the music, or vice versa – otherwise you’ll be left with a audible mess which is guaranteed to be jarring to the listener’s ears.
Consistency is massively not just in the volumes but also in the EQ. Assuming you’ve got good source tracks which don’t fluctuate wildly in the volume, use a bit of audio compression on the voice track in order to bring up the low end and put the voice squarely in the clear and bright higher frequencies.

Additionally, adjust the EQ on the music to around 1500hz before tinkering with the Mid Gain – dropping it by around 5dB is a good starting point. If all goes well you’ll end up with your voice and music tracks occupying two different areas of the sonic range and complimenting each other well, though you may need to use further adjustment of the volume levels after playing with the compression/EQ settings.

For the most part, you’ll be able to tell just by feel as to what compliments a particular voice-over – if it doesn’t sound quite right, or obscures the narration, adjust the levels first of all to rule out any volume issues. If that fails, swap out the music for something radically different and see if you can identify the exact reason as to why the first selection didn’t work. Chances are it’s either an issue with the key or tone of the song, or the EQs clash.
All well and good… but where’s the best place to get free music?

Browsing the McCloud Cloud
By far the best resource for totally free music is Kevin McCloud’s library over at incompetech.com. The site is very easy to navigate, including an in-browser preview player and the ability to search by ‘feel’ (such as comedic, dramatic, epic, et cetera). McCloud is a fantastic composer and the file bitrates are offered at 320kbs, with the file notes telling you at a glance the BPM, instruments used and any notes regarding the key or feel of the song. In addition, while his work is of such a high caliber it is frequently used in professional projects and in top animation schools, the music never gets overused given the sheer size of the library which the public can choose from (over 2,000 tracks to date).
Best of all, all the music hosted there really is free to use with no strings attached – all that’s required is an acknowledging credit back to the site. If in doubt as to how to credit Kevin, a simple “[Song Title Here] by Kevin McCloud at incompetech.com” suffices but if you want the full legal boilerplate, use:

Song Title Kevin MacLeod (incompetech.com)

Licensed under Creative Commons: By Attribution 3.0

http://creativecommons.org/licenses/by/3.0/

If – for whatever reason – you’re not able to credit the source material, a paid option also exists and he is available for personalized scoring. Either way, consider leaving a donation if you find Kevin’s services useful.

What About Sound Effects?
Sound effect files are generally offered under the exact same licensing as general music tracks – in fact, consider them to be the same thing when navigating through the legalese.

A few resources to check out:
Soundjay: A very slick site offering a wide range of sounds, all of which are totally free of charge (and royalty free) – accreditation isn’t even needed, unless you’re using their music tracks. Just don’t host the sound files for download elsewhere, or claim that you created them.

FreeSFX: A niche sound effect library which focuses solely on human voice recordings. They’ve pretty much got everything you could dream up and the library is easy to navigate.

GRsites Archive: There are a good number of totally free sound effects here, handily separated into categories. All files (both WAV and MP3) are free to download and use, although the quality is somewhat lacking and you may have to do some jiggery-pokery in Audacity to reduce background noise.

Final Note: Don’t Try and Cheat
… Particularly on YouTube. One thing we’ve noticed over the past couple of years is the powers that be have gotten very hot on copyright protection, so much so there are many complaints about false positive flagging and blocking of videos which demonstrably don’t have any copyright issues.
Essentially, don’t tempt fate. The last thing you want after pouring weeks of hard work into an animation or film is to have it blocked from anyone seeing it just because you tried to get away with sneaking in a 4-second music clip. It’s a nightmare to resolve things after it happens, and an even worse result would be the sound’s creator catching wind of the discrepancy. Trust us – it might seem like a victimless crime, but you certainly don’t want to be a victim of a copyright claim.