When u export the track down too a wav.form and i convert it too mp3. if i listen too it in headphones i can only hear the vocals in the right earbud.

Its described in the manual https://www.manualslib.com/manual/514807/Boss-Rc-50.html?page=83

You have a memory problem. If you are using a flash card for memory or a CD drive or perhaps you have configured an external drive, you need to free up some space. Try deleting tracks or songs you don't need anymore.

Each song is independent on a CompactFlash
without affecting other songs. You can give a desired song name for managing recorded songs
(see pages 23 and 91).
Recorded track data is stored in a song as mono WAV files.
The MR-8mkII can convert the two WAV files recorded on tracks 7 and 8 (L and R) to a stereo
WAV file (see page 115).
You can export the converted stereo WAV file to your USB-connected personal computer, and
use the file by a music software application or make an original audio CD on the computer using
a CD burn software application. You can also burn an audio CD by using an external CD-R/RW
drive connected via the [USB HOST] port of the MR-8mkII (see page 115 and 123)
Fostex MR 8MKII Owner Manual Page 73 Fostex MR 8MKII Owner Manual Exporting Data To Personal Computer...

Are you using the auto delete feature? Get rid of data you don't need here: http://www.manualslib.com/manual/53326/Fostex-Mr-8mkii.html?page=135#manual

Why you have to convert to mp3 ?. Did not the card can be removed and installed on the computer and then copied?
Where you convert the file? BR or computer?
What software you use to convert?

open up the battery door. Make sure that the switch it is not set to "stamina mode". This disables many main menu options

They make headphone amps that will take a line level and boost it in volume until it can drive headphones. I'd get one of those, and plug it into any available "line out" or "rec out" on the back of your Fostex. Sounds like the D/A converter section that feeds the headphone output has gone bad, and my solution will get you around that.

Make sure that the file format is the same, first. If so, try converting the format from wav. to a more recent format type, such as mp3, or mp4 if possible. This may help, as the computer may recognize it better depending on the program you are playing them with.
Please let me know if you need further assistance.
carsandcomps,
Guru, Premium Expert, Phone Support @ Fixya.com

It’s necessary to bounce all of your tracks to a stereo pair of tracks before you can export your song to a computer as a WAV or MP3 file for burning onto a CD.
1. In normal mode, push EXIT and UTILITY until you see BOUNCE on the display.
Note: The upper right corner of the display shows the track you’re bouncing to. By default, “12V2”—Tracks 1 and 2, Virtual Track 2—is selected.
2. Press STOP and REWIND together to go back to the beginning of the song.
3. Press RECORD, and then press PLAY.
4.When the song’s finished playing, press STOP.
i read some stuff online and this should help.

Go to bossbr.net - Plenty of answers there! And yes, you have to master or export the tracks to .mp3 or .wav. These files will be located in the mp3 folder. Boss also makes a free .wav converter located on their website.

The autoplay window should have popped up, and then select "Open folder to view files". If not, open file explorer and there should be a drive labled "Boss_BR". The only playable files will be located in the MP3 folder. You have to export your tracks (Using the MBR) as .mp3 or .wav for them to be playable.

from reading instructions when you record channel 1 and 2 you are recordind channel 1 in stereo so you only recorded channel 1 try rcording on chanel 3 for vocal hope this helps