All right I guess now that I have a moment, I'll update with my thoughts regarding my favourite girl!
I don't think the Rockin' Beats will stand the test of time - they'll last until high school (with maybe one departure before then, likely due to someone moving) but then once they hit their teenage years, the band will dissolve. Below is my personal version of what happens.
Melody - will continue on with music, going solo for a while (more about her music below)
Tunefull (the keyboardist) - she moves away in junior high, which prompts the beginning of the end of the group (Tunefull was my "moderator" in the band, who would make sure the other girls never fought too harshly with each other; when she leaves Melody plays the keyboards in her absence when necessary)
Pretty Beat (the drummer) - she veers away from the other girls rather quickly in high school (she was always into hard rock more than the others, and as a teenager she gets into the goth/metal scene)
Sweet Note & Half Note (guitarist and bassist) - in my world these girls are twins, so they always side with each other (while Melody and PB would side against them, which is when Tunefull would step in); once they reach high school they get way more into pop music and actually want to change the direction of the band drastically towards more of a pop girl group where they can sing and dance instead of just playing their instruments (they basically start feeling like Melody is always stealing everyone's spotlight)
So yeah - in high school the twins get so fed up with not having more of the spotlight on themselves, and with Tunefull gone, everything falls apart. The bust up is actually rather harsh - Melody and the twins do not remain friends, while PB doesn't really want anything to do with all three of them at first (though she and Melody get back together again later). The twins also join the cheerleading squad at school (because cheerleaders do exist in the comics!) and from that they create their own girl group with two of their cheerleader friends. PB is in another band (their first incarnation is called Bored By Noon, where she plays drums, an OC of mine called Princess plays guitar and sings, and there are two other members who I actually never got around to fleshing out, oddly enough, but they were guys) and Melody is just focusing on writing her own music and taking a bit of time off from performing (she uses the time to improve both her guitar and piano skills).
After a year of not performing though she wants to start it up again (at this point she and Ace are dating, and Ace is also friends with PB and Princess). Melody and PB still haven't really spoken since the breakup, but Ace gets the girls chatting again, to the point where Melody starts showing the other girls some of the stuff she's been working on. After the year apart Melody and PB actually realize how much they miss each other, and PB invites her to join the group. They change their name again and shuffle their style around (making music that's a bit lighter and dancier than just harder rock/metal), and eventually end up with something they all really love. They battle it out in school talent competitions and band wars with the twins and their group! The four original Rockin' Beats never really put things aside, so they battle all through high school.
So having said all of that, I thought I'd share some of my inspiration for Melody's music. I'll admit that I've totally brought them into the current music scene, though I do want to head back to the 90s to find a band to inspire them at that time period (off the top of my head I'm thinking I might look more into
Republica). I have a few different music muses for Melody, but as far as her post-Rockin' Beats, high school band with Pretty Beat goes, I like to imagine them sounding like one of my personal favourite bands, The Birthday Massacre (they'd have a different name and slightly different image, though). In particular my favourite muse songs to fit them would be the following:
Always - A song about a failing relationship - which Melody has written about the Rockin' Beats. It's all about the group dissolving, though when they were younger they'd all promised that they'd never split up, as kids always do.
In The Dark - Another song about her band breaking apart, and feeling alone even though she was supposed to be with friends (basically talking about the moments before they broke up, and were constantly arguing with each other about their music).
To Die For - This one is about Ace (of course I couldn't list songs and not have a relationship song!); they break up at the end of high school because he moves away to play soccer professionally, and she doesn't want to leave her band. So this one is about that, and dealing with her feelings regarding letting him go.
I Think We're Alone Now - This song doesn't really have a particular meaning to them, I just love the original and it's always been one of my personal Tales songs, so the fact that this band has covered it is definitely something that fits my muses.
Anyway I could go on with songs, but those ones are a good representation of what I imagine their band to sound like.
I also have a pop-based Melody, who just sticks to being a solo artist after the band splits up. Originally I drew inspiration for her from Katy Perry, but now my ideal sound is Allie X -
Catch is the song that drew me in. Her stuff though is more suited to a slightly older Melody (thinking post-high school, university-aged).
I'm a big metal fan myself - my favourite genre is actually black metal (my favourite band in any metal sub-genre is Dimmu Borgir). Big fan of Slayer (they are amazing live!) and Fear Factory (who also put on an amazing live show). And speaking of grunge and therefore Seattle - I also really love Soundgarden (got to see them live three times in the span of last year; and walked away with Chris Cornell's set list at one of the shows!). Curious to know who you're into! 
