Everything

If my memory serves me correctly, the fastest way to grind audio clips was the following:

-1.) (optional) Ascend to the Planetary level.
0.) (optional) Choose another planet to descend into. Make sure it's one that would typically have continents.
1.) Ascend(/descend) to the Continental level.
2.) Choose a continent that you have not been on yet, and descend into it. (If you just jumped from another planet, you can skip this step.)
3.) Ascend into the largest thing available. Whether that be a tree, a windmill, a building, or something else.
4.) Scale yourself up (r) as much as you possibly can.
5.) (optional) Hold left click, and descend into anything you don't have yet, then ascend back into the largest object you once were. This is very helpful for collector achievements like One Million Heartbeats. Heck, i even unlocked some new animals while i was re-testing this route!
6.) Look and move around (not for too long, the world loops over quite quickly - if you descend you may be able to see multiple versions of yourself in the distance) for a little circle icon. I, unfortunately, forget exactly what it looks like, but i recall it being obvious.
7.) If anything has a circle icon, get closer to it and then press (x.) 1 audio clip unlocked!
8.) Repeat from step 1 (or -1 if you've run out of continents.)

If anything in this guide is wrong, let me know - it's been some time since i got 100% in this game, and this was written mostly from memory.