With the different audio and video formats available, there is often
the need to inter convert amongst them - sometimes for quality and
sometimes for compatibility. Here are some of the better software, that
you can use to achieve the inter conversions on your Linux box.

Available via the package manager, Sound converter provides basic
batch audio file conversion. Select the files or drop in an entire
folder, choose the output format and bitrate from within Edit >
Preferences and basically, you’re done.
Gnormalize

Gnormalize is a GTK based tool for audio conversion. In addition to
converting audio (between mp3, mp4, mpc, wav, ogg, ape and flac),
Gnormalize can adjust the volume of sound files to compensate for
varying recording levels. You can also use Gnormalize to rip CDs, edit
metadata and play your songs as well.
SoundKonverter
KDE users can try SoundKonverter. It has all the features of
Gnormalize like reading tags, replay gain calculation but supports a
few additional audio file formats.
OggConvert, WaoN and flac
You always have a plenty of choices at your disposal when choosing
software in Linux. Here are some command line tools that are good for
specific audio conversion tasks. OggConvert provides you tools to
convert almost all major audio formats into Ogg. flac and WaoN are good
to use use when working with Flac or mid files respectively.
SoX
Then there is SoX - Sound eXchange. Although not just a conversion
tool, geeks swear by it. You just cannot write an article about sound
and not mention SoX. It does some hundred different amazing things and
is rightly called the “Swiss Army Knife” of sound-processing programs.
WinFF
FFmpeg pretty rules the roost here.
You can get all geeky and learn the command line switches or you can
try WinFF. WinFF provides a frontend to FFmpeg. It works on Windows and
Mac as well. It (actually FFmpeg) can be used for batch conversion of
audio and video files. Just add the file(s) you wish to convert, choose
the desired format, apply device presets if you desire. Once you are
set, hit the Convert button and out pops the command line with one big
ass command! Glad we don’t have to type in that ourselves, thanks to
WinFF.

You can do pretty amazing things with WinFF or FFmpeg in general. We saw how we can use it to create actions so that you can convert video for your iPod with a right click in Nautilus.
Studying the command line that WinFF pops out, you can create more of
such custom actions. You can for example, write an action to extract
audio out of videos, the possibilities are endless.
Handbrake

Handbrake is a popular multi-platform video transcoder. It can be
used to convert DVDs to MP4, MKV, AVI and OGM. It offers additional
features like chapter selection, burning subtitle into the picture,
cropping and scaling.
DeVeDe

If Handbrake converts your DVDs to MP4s, DeVeDe takes in video files
and creates DVDs and CDs that you can run on your regular home CD/DVD
players. DeVeDe is available for Windows as well.