converting BUN files to MP3

You'll need to open the BUN file into Cakewalk, export to wav file, then convert the wav file to mp3 using a program such as THIS free one.

Are you using Cakewalk Pro Audio Deluxe Ver. 9 ? If so there is a Fraunhofer MP3 Encoder built in that works like a champ. For Profession Output Results:
Go to
Tools
> Mixdown Audio
> Export to File(s)
>(Mixdown Audio/Export to File(s) Dialog box appears)
> Click on "down arrow" on "Files of Type" and select MP3
> fill in filename (give the recording a name)
> Click on "Export"
>(At this point the software exports the file to "audio" ; You will see the load bar grow across the bottom of your screen.)
When it's complete, a new dialog box will appear: "Cakewalk MP3 Encoder" Information in the first two fields are "Wave File Name (Input)" and "MP3 File Name (Output)". This information appears in the 1st two fields but is greyed out)
> Select "Sampling Rate" = 44100
> Select "Bit Rate" = 320000
> Stereo Mode: Select Stereo
>Optimize Encoding bar should be maxxed out to "Quality"
>Click on the box for "Include ID3 Info: Click on "Set ID3 Info"
> Fill In Title, Artist, Album, Year, Track No., Comment, Genre, and click on OK
> Click "Encode"
Your audio file will now be exported to a MP3 file.

I would suggest to burn this to a CD now, and then rip it into Itunes, but make sure in your iTunes Preferences, you have chosen Bit rate of 320K and Sampling rate of 44100.
This will give you the highest quality MP3 output of your conveeted cakewalk .bun file.

Note: For web posting or emailing of mp3 recordings, I would suggest lowest bit and sampling rates to keep your files small. The above instructions are for Cakewalk Pro Audio Deluxe Ver. 9 Wich was an excellent version with all the bells and whistles I would ever need, right before they went to Sonar. I have recording numerous band and produced hundreds of CD's with this version.