online media file converter may be the easiest way as it works whether you have a Mac or Linux or indeed Windows - including if you just want to extract the audio sound file from a video on YouTube or another online video service - though if you don't have broadband then obviously it may be a bit slow.
(I've posted before on how to extract MP3 or other sound from video files but that involved only Windows-centric software, and the process can be a bit involved.)
Here are two free online media file conversion services I've tried and suggested to other people who've tried them too, so I know they work.
With both of them, you can enter a YouTube or other video service link to convert the video direct into MP3 or other file formats over the internet; or alternatively you can upload a file from your own computer for conversion and download the converted video, audio or other file.
Media Converter say they support audio formats like mp3, ogg, wav, wma (only decoding supported), and video file formats like 3g2, 3gp, avi, flv, m4v, mkv, mov, mp4, mpg, mpeg, psp (only encoding supported), rm, and wmv, though I've only tried the flv to MP3 conversion myself. (They also say they convert Office document files i.e. text / wordprocessing and presentation documents, spreadsheets etc: doc to odt or pdf, xls to ods or pdf, ppt to odp or pdf, odt to doc or pdf, ods to xls or pdf, odp to ppt or pdf).
It's free only for file sizes up to 100MB, and you pay for the premium service for bigger files.
Media Convert are more mobile orientated e.g. their "Send File to Mobile" tab enables upload of a file for download on a cellphone, and they have a dedicated mobile host for video or music files.
Supports a wider range of audio files (I won't list them all, see their list of supported file formats, but they include ringtones and .amr and 3gp files as used on mobile phones) and office file conversions (including html, Open Office files, Star Writer etc) - and also compressed or archived file formats like zip, rar, lzh, tar.
Will convert image files i.e. pictures, photos of different formats including gif, jpeg, png, with the ability to resize and compress them, and produce a screenshot of a webpage in your chosen file format (yes you could use your Print Screen key; but it just saves time with converting the resulting file). It even converts pdf to Flash swf.
It will also create a free webpage (with ads, not surprisingly) with a streaming version of the converted file, which you can link to - though I've found it a bit variable as to how well that works. If you want to download the converted file, do that before you try the webpage option, or else it'll vanish and you'll have to convert it all over again.
Up to 150MB size of files converted. Ad-funded. (600MB for their mobile video or music files.)
Personally, I generally prefer the latter as it provides more options (see pic above) in terms of both file conversions (e.g. it will convert PDF source files whereas the former only converts certain formats to PDFs) and ways of presenting the converted files. But as mentioned the webpage streaming option seems not always to work. They're free, so try them both and see which suits you better.