READ ALL HINTS BEFORE STARTING
Only ask questions which have sense, i will not answer stupid questions

Tools you need...
NHL07AsfStudio - The tool for packing and unpacking xditties.ast
fAtWab - Tool for converting ASF to WAV, WAV to ASF and MP3 to ASF
Audacity - Tool for editing sound files (remember to change to 16-bit in program options)
MP3Gain - Tool for setting all mp3-soundfiles to the same volume before editing
xditties Documentary - Table about contents in xditties.ast (not up to date, due to a data crash i had)
Short info:
1.) Copy xditties.ast and xditties.abk to a desired working-directory
2.) Start NHL07AsfStudio, click on unpack and choose file you copied on last step (in working dir)
3.) Wait
4.) Now you habe all files in one directory (sub dir)
5.) Use my Table to find out what is what
6.) Look at the duration and think about which music should be replaced by what
7.) After you found some mp3-files which you want to use for replacing use mp3gain and let them be adjusted to one level
8.) Start Audacity and go to options. There you set to 16-bit
8.) Now open the mp3s with audacity, cut them and maybe add some echo (good options are 0,2 and 0,3 i think, if not try...
9.) After that export them as wav and rename to the same name which the music that has to be replaced has.
10.) Use now fAtWab to convert from WAV to ASF.
11.) ...
12.) After you've done that replace files in the unpacked directory with your's and click in the ASFStudio on Pack and choose directory where you replaced the files...
Done
Extra hints:
1.) For introduction music you should not exceed the duration of the original, because then music will continue playing after the game started.
2.) Make backups before replacing xditties.ast and xditties.abk
Have fun