I think it would be very cool to be able to access all of the features from one screen. However, there are several reasons why it might be better to use a main screen instead.
1. One of the main goals you guys have said you have is the ability to easily add new features via a SDK. If most of the other features are accessed from a home screen then it is easier to add new features. For instance you don't have GPS listed in your layout above. If I wanted to add a GPS plug-in and you have it organized the way you are planning I would have to edit the MP3 playback screen instead of just adding a new icon to the home page.
2. Most of the time people are going to be using this software to play MP3s and the number of times that the average user is going to change songs far exceeds the number of times that user is going to switch to and from video playback or one of the other features. Thus, it makes sense to make the MP3 screen more devoted to the playback of MP3s then to other things. I would recommend, and obviously this is just my humble opinion, making the browse window as large as possible so it is easier and safer to change between songs while driving. Increasing the font of the songs on the list would make it easier to hit the correct song/folder while driving on the small 7" screens that most of us are using.
Maybe something like this, I did it in paint so it is very rough. I don't know, its just an idea and obviously it's your program. Thanks for all the hard work.
oops, forgot to leave the browse button in.