You want to split a AVI file into audio and video file and then merge them into a Video AVI file?
WindowsAPI supports Avi file, it's a good news for you. But it only can render bitmap frames, and no sound playback. But, editing AVi files using Windows API is very nice! Can merge, can split, can mix, can insert, I think it should suffice, shixiyu123. You know Windows API programming? If so, you should learn & study a few basic lessons (can be found in Microsoft). This includes : Some related multimedia categories (sound, video, music, windows), important functions (can be found in Microsoft samples or examples), and how to apply it for your projects through Ex & samples
Other method that isn't related to API is DirectX (DirectX Sound, DirectX Video). DirectX is used to create almost of famous and popular games. You know DirectX Programming? And DirectX SDK?
Long story short, your idea is very good. It's better to switch your post category to Windows Programming, because you may get more replies and helpful advices from others and experienced programmers...
If you need help, please PM me and I'll give you more details.