So I've been reading a lot of IDW Transformers comix, but I have yet to come across the answer to this:

IDW Soundwave clearly has all the kibble-y bits and deco of an Earth music player. Have they explained how that happened? I read something recently about him being stuck in his alt mode. What's the story there?

Thanks in advance!

 

Basically, during Spotlight: Soundwave it's explained that he's been keeping tabs on Bludgeon for Megatron.
He follows Buldgeon and his team to Earth.
Some stuff goes down and Soundwave gets shot with an experimental weapon, thus getting trapped in his cassette-player-mode for a good few years.
For a slightly more concise version, click on this here link: Spotlight: Soundwave - Transformers Wiki

 

Ah, I remember those stories. They were my first foray into the comics; I had gotten the Heroes and Villains paperback book from a school book fair, and fell in love at once.  
But on the topic at hand: if I'm reading what you're asking correctly, grindcore138 is right. Are you wondering when he originally became a tape player, or just why he has the kibble of one?

 

Thanks, GC138. I've yet to find that comic.

The cassette recorder seems pretty literal to his mission here, but that's TF for you. Is there any explanation as to why he kept that body? Aftereffect of the experimental gun?

Your understanding is right on. I do think it's weird that he seems to have kept the look, though.

 

Because maybe he likes the look of music player and tries to ensure he still on his body.

TFA Soundwave has that too.

 

No real explanation. By the time he rejoins the rest of the 'Cons, it's during All Hail Megatron, which has a serious G1 Cartoon aesthetic. Not that there's anything wrong with that.

 

Well his role doesn't really require the use of an alternate form. There wasn't really a use for disguise in AHM and as communications officer he isn't exactly rolling into the heat of battle and tearing shit up, he's more or less taken the sidelines for the most part, letting Frenzy or Ravage do their thing while he lurks in the background. At this point he's since been reformatted into a van, and seemingly again into a military vehicle of sorts in order to blend in again momentarily.