i do feel that avidemux is the perfect tool for this trimming job.
here's a quick'n'dirty tutorial.
for example, here i have a neil young video from 1972 (?). let's say that i really don't like the first song, and i want to delete it.
i would load the video file into avidemux, then fast forward (by using the slider or arrow buttons) to the end of the unwanted portion - see first picture.
i would then press the B button - this sets the endpoint of a selection. i didn't need to press A (startpoint), as it assumes that A is the start of the file. if i wanted to make a selection in the middle of the clip, i would press A at the startpoint, then B at the endpoint.
ok, now that i have my selection selected, i simply press the delete button on my keyboard.
job nearly done. i just have to save the movie. it will not get re-transcoded, meaning you will not lose any quality. it will keep the same exact video format as the original.
one thing to note is that the program doesn't add the .avi file extension - so make sure you type this in when you save.
unfortunately, avidemux will only work with video files. to edit your mp3s you will need an audio editing program. audacity
is a good freebie,a nd simple to use.