What I mean is a Behemoth-sized biter sounds like a Small Biter. Is this intended or an oversight?
No matter what size a unit is, to me, they all sound like the smallest version of that unit (biters and spitters). I don't know if this is true for worms.

I also don't know if anyone else brought this up at some point. I did a quick search on the Github repo for "sound" and "sounds" but I didn't see a mention.

Do they have different war cries and songs? :-) kidding. I didn't know that, probably because don't remember vanilla biters.
No one mentioned it. Are you playing with New Enemies mode checked on? Much more sizes and many tiers, than vanilla Factorio has. Probably Rampant shouldn't change sounds when playing without new enemies.
Is the same thing actual for worms too? Is it just sound volume or really different sound packs?

Except for acid-mid and acid-end in the sounds folder, everything else sounds like the vanilla roars and attacks. Bigger units just have louder versions of their small counterpart (behemoth biters sound like small biters, just louder).

Also, when playing around with worms, I noticed the animation where they go back into the ground skips several frames.

I'm modifying a mod called Darkstar Utilities that I guess focuses on being crazy (dyson spheres and interstellar dominion, etc). it allows the player to craft units and place them down as allies. I'm adding support for Rampant units so I'm noticing all these things.

I neglected to answer one of your questions. >.< Yes I have New Enemies on. It sounds like only the volumes of small units are being adjusted for the bigger units.

The upcoming version should fix the missing and incorrect sounds on new enemies.

Some distinctive sounds did really appear... but at least for me this fix didn't go well.
Factorio engine doesn't support it
https://forums.factorio.com/viewtopic.php?f=49&t=101651

I posted a question about this problem there, but my post was "moderated".........