Did you open synaptic and install 'soundconverter' yet?
If you're using a gnome desktop, then you want soundconverter - but for KDE freaks there's 'soundkonverter'
You could just do it with a script for nautilus (see 'sources' below)
In fact, there are so many different ways of doing this that I can't imagine why you'd ask.
Do you know how to find the synaptic package manager?
you can launch it straight from your 'System Administration' menu in the panel, or you can press ALT F2 to get a launcher and type 'gksu synaptic' (you need to type gksu because it must be graphical super user)
You can open a terminal and type 'sudo aptitude install soundconverter'
If you don't want to type 'sudo aptitude install' then you can edit '.bashrc' and put in 'alias install='sudo aptitude install' and while you're there put in
alias update='sudo aptitude update && sudo aptitude upgrade'
## Sudo fixes
alias install='sudo aptitude install'
alias remove='sudo aptitude remove'
alias autoremove='sudo apt-get autoclean && sudo apt-get autoremove && sudo apt-get clean && sudo apt-get remove && orphand'
alias wget='wget -c'
alias gsyn='gksu synaptic'
If you do that, then just do ALT F2 and type 'gsyn' to get Synaptic. Search 'soundconverter' and that's it.
If you use Ubuntu for a year, you'll have so many ways of doing things you'll be amazed why it was ever difficult.
I'd fully recommend you get up to 9.04 now, it's sooo much better than 8.04 - and if you install from the CD, you can reformat to ext4, which is faster and better...