if you have the video, then all you need to do is add the audio. there is NO all-in-one solution for this (not for free anywayz), but if it is an AVI file, you can use VirtualDubMod to add audio. if it is WMV or MPEG, you can use Windows Movie Maker. if it is MP4, you might have to convert it first.
for audio editing (trimming length, etc.), you can use Audacity, which is the best FREE option there is.

Non-free solutions:
Sony Vegas is the best at layering videos and audio files, and is the "weapon of choice" in the audio/video editing world.

Womble MPEG Video Editor can add audio without re-encoding the video (which is important for hi-quality video).

TMPGEnc is my personal favorite video editor that can save files in a HUGE variety of formats, and can OPEN even MORE formats for conversion. it also has a variety of video editing options, but has very little in the way of audio editing, though you CAN combine audio and video, but it WILL re-encode the audio and video, losing a hair of quality.

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