If you are getting troubles installing ffmpeg with sound support on your ubuntu machine then follow this guide. I am pretty sure this one is quite easy to follow. 😉 1. First of all install php5-ffmpeg via apt-get.
sudo apt-get install php5-ffmpeg
2. Now add mediaubuntu repositories:
sudo wget https://www.medibuntu.org/sources.list.d/hardy.list --output-document=/etc/apt/sources.list.d/medibuntu.list
Then add GPG key:
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update
Source: https://help.ubuntu.com/community/Medibuntu
3. Install some codecs
sudo apt-get install w32codecs
4. Install some libraries
sudo apt-get install libavcodec1d libavformat1d libavutil1d libc6 libfreetype6 libimlib2 libsdl1.2debian libswscale1d libfaad0 amrnb amrwb
Now you should have support even for 3gp video files and sound suport ofcourse.
5. If you still have problem converting videos with sound support try to reinstall ffmpeg:
sudo apt-get install --reinstall ffmpeg
Good luck!