ok heres what you do, get the x files theme tune (doesn't matter what format or bitrate or length) then download and install a program called goldwave digital audio editor (fully functional evaluation version)
http://www.goldwave.com once installed download the atrac3 codec
http://www.softpedia.com/get/Multimedia/Audio/Audio-Codecs/Sony-ATRAC-Audio-Codec.shtml and copy the contents of it to the goldwave folder and run it.
once that is done download the psp bootsound replacer
http://www.savefile.com/files/1446424then run goldwave and open the x files theme song, trim the song to the part you want (it must be between 1 and 4 seconds long) then save it as a .wav format (under attributes choose "atrac3 132 kbps stereo")
then close goldwave and open psp bootsound replacer select the firmware you're on (only works with firmwares 3.71m33, 3.80m33, 3.90m33) and click open WAV and select the file you created using goldwave then on your psp, go to recovery mode -> advanced -> toggle usb flash0 (remember to backup everything you mess with). then on psp bootsound replacer save it in X:\vsh\module , it will overwrite the original. and you're done
here is a video on how to do it
I TAKE NO CREDIT IN CREATING THIS VIDEO OR ANY OF THE SOFTWARE OR UPLOAD LINKS I SIMPLE WATCHED THIS VIDEO AND CREATED A BRIEF TUT, ALL CREDIT FOR LINKS GO TO THE AUTHOR OF THIS VIDEO
P.S backup everything you mess with, and also again this only works with firmwares 3.71m33, 3.80m33, 3.90m33
http://www.youtube.com/watch?v=Um6RyQgeIfY