Mp3 extraction with avconv or ffmpeg doesn’t work properly anymore

For editing mp3's under linux, I'd recommend sox. It has a simple to use trim effect that will do what you ask for (see man sox for datails - search (press/) for "trim start"). Example:

sox input.mp3 output.mp3 trim 1 5

You didn't mention it, but if your aim is just to remove the silence at the beginning of files, you will find silence effect much more useful (man sox, search for "above-periods")

The final line in the output log is too generic — you'll have to look up further. The error is this:

[ac3 @ 0x9fdc740] invalid bit rate

The problem is that for some reason avconv wants to encode your MP3 audio to AC3. In fact, when you're only resizing video, you can leave the audio bitstream alone.

Note that resizing and re-encoding will lower your quality drastically. So, unless you really need to, don't do it. Your video will suffer from generation loss.

If you can, avoid using MPEG-4 Part 2 codecs (Xvid, or the libavcodec-native mpeg4), and use MPEG-4 Part 10 / H.264 codecs instead (e.g., x264). Since H.264 isn't properly supported in AVI containers, we'll use MP4 instead, which should be your container of choice instead of AVI most of the time.

ffmpeg -i in.avi -c:a copy -c:v libx264 -crf 23 -s:v 640x360 output.mp4

This will copy the audio stream (-c:a copy), encode the video to x264 (-c:v libx264) with a constant quality of 23 (-crf). Use a lower value here for better quality (with sane values from 18–28). The size will be changed with -s:v.

I'm using FFmpeg synonymously with Libav here, since the syntax should be the same. I would however recommend you to ditch the default Libav version that ships with Ubuntu and compile FFmpeg from source or use a recent Linux static build.