I need a Audio Enhancer , please help me 10 points?

it is getting on my nerves, I have been on the prowl on the net for 3 hours looking for something, I have a MP4 file, it is a video and I just want to enhance the sound, save it, and upload it to youtube as a video with enhanced sound... Audacity wont work cause it extracts the audio, then I have to piece the video together again in synchronization. Quicktime did what I wanted it to when I went into A/V controls and I saved it, but only works when you play the video IN quicktime. I just want a FREE program or something that can let me enhance/adjust the audio without stripping away the video.. I have a MAC OS X 10.4...