Yes... Sound effects and music are often overlooked by the player since it is something that is more in the background. However, music really does set the tone in a game. Good music can make or break a game, without the player even realizing it most of the time. Customizable music might be good... But IMO not really worth the effort. Most people just run I-tunes in the background if they want different music, so a built in system would be more hassle than it is worth. Especially since you would have to navagate around your computer for various music files, making opening up I-tunes much more simple.

I would think sound effects will be customizable. Just slip in a few extra sound effects, edit some script in your custom model telling it where its sound effects are, and your done. Or for an even easier method, back up the unused effects and replace them with your new ones under the same name.

If dev time were pollen-plentiful (spring has sprung down here in North Florida), I'd want a really good improv system instead of a score. No matter how much I've liked some of the music for a given game, I've always ended up running my own tunes separately because I get a bit tired of the same several tracks over and over again.

Then again, storage is cheaper all the time and MP3 files don't look nearly as bulky as they used to, even at a decent bitrate.

Semi-seriously, I'd like to see most of that audio work for Elemental focus on ambient sounds. I think it was the ambient stuff in HoMM that set the hook for my mother's addiction to the early part of the series--she and I both thought it was really neat that you could hear things on the map as you moved a unit closer to them. So I'd really enjoy something like different hums for each shard type, some woodsy sounds for forests, some good running water stuff, etc. It would also be neat if ambient sound for settlements reflected the size and/or traffic levels.

Well, dynamic music is just a step or two beyond the music in most games, where in a big battle the music gets more epic. For example, in SoaSE if you zoom way out or are looking in a peaceful well the music is much calmer. In a big battle, the music shifts dramatically and becomes much more fitting to that setting. Dynamic music just takes that one step further and has different parts of the music change according to different variables rather than the whole of the music changing based on a single variable. The biggest challenge is to make it all sound at the least decent together.

I think it is important that Elemental has very "strong" music.  Many games just have background music that you barely notice.  If you are managing your civilization or whatever and not actively fighting a battle, some good music will help keep you interested.

Improv music is ok but in my experience it hasn't been particularly impressive.  The iMuse system that Zubaz mentioned sounds like a good idea .