Depending on the type of video and audio (avi, mp3?) I would be inclined to demux the audio (separate it from the video);adjust the volume using an audio editor like Audacity, then join the video and new audio.

To demux the audio from the video, run AviDemux and load or drag the video in.
On the top menu, click 'Audio' > 'Save' and (assuming it's mp3) save it with the complete name 'clip.mp3'.

Once you have the audio, run Audacity.
Before continuing you have to make sure you have the required codec to work the audio.
Click 'Edit' > 'Preferences' > 'Libraries' and locate or download the mp3 and ffmpeg libraries.

Load or drag in your audio file and select the area you want to modify.
Click 'Effect' > 'Amplify' and accept the default.
If satisfied, click 'File' > 'Export' and save the new audio in mp3 format.

Run Avidemux and load the video.
Click 'Audio' > 'Main track' > 'Audio Source, external mp3' and select the new audio.
Click 'File' > 'Save' > 'Save Video' and give it a complete name like 'newclip.avi'

http://www.videohelp.com/tools/Audacity
Google umpteen tutorials for 'Audacity'.

Originally posted by kindachi:

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Thanks attar. I selected the section but after clicking amplify, it highlights the whole song for me :s

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As a heads up, the rules state no posts for video editors.