Tim's right... mostly. He's right in that the music will still mute (unless you have the nav set to "ATT" - then the music will just get quieter). Here's how it works: you can specify how loud you want the nav voice to be in Settings. So when you are listening to AM, FM, iPod, disc, or sat radio, the nav voice will operate at the volume that you told it to be. ...but when you are listening to USB or SD, the volume of the nav voice is tied directly to the volume of the music you are listening to from the USB or SD. I have heard this best explained because the AVIC is running WinCE, these two 'devices' are essentially being played from the native Windows functionality instead of a standalone application - and so is the nav voice.
Where I disagree with him is where he says that the nav through USB/SD sucks. Sure, it could definitely be better, but somebody on this board said it best when they said that "there's no VR control over SD or USB or CDRW because the AVIC has no idea how you sort your music" - so when you want Linkin Park, the AVIC has no idea how deep in YOUR CUSTOM folder structure the artist's name is. Personally, I do everything through Windows Media Player and Vista's Media Center, and my music is all sorted in nested folders much like they exist on an iPod anyway: Artist, then Album, then song title. I took the 'data' that is in the "My Music" folder in Vista and just burned it to a CDRW, retaining the entire folder structure. So when I want to hear Blink 182, I go to the 'B's. When I want to hear Linkin Park, I go to the 'L's. Once I'm in the artist's folder, I just select the album and then play the entire folder. (Or you have the option to play random throughout the entire disc).
Things that I wish the AVIC did for USB/SD:
1. Ability to play from a parent folder. If I have 3 Weezer albums and I want to shuffle all of them together, I can't do this with my current folder structure. I would have to eliminate the album branch altogether to get all of my Weezer songs in the same folder.
2. Easily get to the next SHUFFLED song. If you've enabled shuffle and it takes you to a song you really wish you had forgotten about, you can only hit "Next" or manually specify a new song altogether. But when you press "Next", it just goes to song right after the one that was already being played - it DOES NOT pick the next random song.
I hope this helps you out. Personally, I'm not a developer (I actually work in QA) but if I know developers at all, you'll like this unit. Being from a QA background, I *do* find that they perhaps let this thing out of the box a little early. If you need any further information, just lemme know.