Original Tutorial by ChuckiePK from the Nextwave 64 message boards
THE PROGRAMS NEEDED
Download these programs:
THE PREP WORK
Get all the songs from the rom by nomercymusic.rar by downloading this zip file
or
Open N64 Sound Tool (under N64SoundListToolUpdated/release/N64SoundListTool.exe) and find the rom you want to use

Choose the last soundbank (soundbank 02)
Change "Master Sample Rate" to 7000

Save all the "Instruments" (under the sound box) one by one, each "instrument" is a music file for the song.


Music listing for No Mercy (with time lengths)
0000 - 0:46 - Kane
0001 - 0:41 - New Age Outlaws
0002 - 0:28 - Shamrock
0003 - 1:19 - Venis
0004 - 0:24 - X-Pac
0005 - 0:29 - Al Snow
0006 - 0:23 - Bossman
0007 - 0:10 - Blackman
0008 - 1:19 - Vince McMahon
0009 - 0:45 - D'lo Brown
000A - 1:02 - DX
000B - 0:49 - Edge
000C - 0:48 - Godfather
000D - 0:25 - Mark Henry
000E - 0:10 - RAW
000F - 0:47 - Mr. Ass
0010 - 1:02 - Hardys
0011 - 0:49 - Hollys
0012 - 0:50 - Dudleys
0013 - 0:21 - Bulldog
0014 - 0:03 - Viscera
0015 - 0:20 - Essa Rios
0016 - 0:46 - Chyna
0017 - 0:32 - Malenko
0018 - 0:35 - Taka
0019 - 0:27 - Tazz
001A - 0:51 - Patterson (Real American)
001B - 1:17 - HHH
001C - 1:05 - Stone Cold
001D - 1:35 - The Rock
001E - 0:33 - APA
001F - 0:24 - Mankind
0020 - 0:44 - Too Cool
0021 - 0:38 - Cactus Jack
0022 - 0:38 - HBK
0023 - 0:05 - Chris Jericho (intro)
0024 - 0:42 - Chris Jericho (the actual theme)
0025 - 1:03 - Angle
0026 - 0:30 - Benoit
0027 - 0:18 - T&A
0028 - 0:08 - Guerrero
0029 - 0:17 - Saturn
002A - 0:02 - RTC
TIPS FOR MUSIC CHOICES
Since at the end it is best to get as many themes you can in the final step, some prep work should be kept in mind when choosing themes. Keep in mind that the themes that are added in should be a little bit less than the one they are replacing. A great tip to keep track is to have all the original music files in one directory, then organize them by using the length option in the menu. So for example if you have a file that is 43 seconds long, it would be best to set it for a space that is 44-49 seconds long.
Keep in mind that you will probably only hear about less than a minute, so the whole song isn't necessary. All the songs require a loop which can be placed anywhere in the song. So the majority of themes would consist of an intro, followed by a repeated riff. As long as the general feel is there, then it will usually work.
Also consider looping a short riff or a beat. There are some themes that are just 2-5 seconds long and consist of a loop (like Right To Censor's alarm, Viscera's beat, etc.) There are a lot of songs there with repeated riffs that could be used since they are so short (like Bret Hart's riff, The Nasty Boys' drums, Earthquake's drums and rumble).
A good way to deal with the limited amount of songs, is to use one song for as many characters as you can. This works well for heel stables, characters with a general theme (patriot, hardcore, evil), or just a song that would fit for several characters.
Theme 0023 is unique because it is tied to Jericho's titantron. It is just a 5 second loop for the computerized voice during the countdown. However if you pick any character and give them that titantron, then it will play that sample twice before going into your regular theme. Perfect for a heel stable intro ("New-New-New-World Order", "Bullet Club for-for-for life").
AUDACITY
Open Audacity, and drag and drop your wave file from the N64 game on it FIRST.

After that you drag and drop your mp3 file on the same project. This will preset the sound of the new mp3 that you drag in so there would be little trouble when it is finally injected into the new rom.

Now cut your new theme, I recommend cutting it a bit shorter than the older theme so you don't have size issues later.


Remove the older track
"Tracks -> Resample" your new theme to 7000Hz.


Then "Tracks -> Mix both stereo tracks to mono.

Your track should look like the last screenshot. Export it as WAV, signed 16 bit PCM. (A good thing to do is to save the file using the "instrument" number from the list for easy reference later.)

WAVOSAUR
Open your new wav file on Wavosaur
Highlight the part you wanna loop.
Press SPACE to preview the looping part of the theme to make sure the loop repeats as seamlessly as possible
Go to "Tools -> Loop -> Create Loop" then press Ctrl+S to save.


N64 SOUND TOOL
Open N64 Sound Tool again
Choose the last soundbank (soundbank 02)
Change "Master Sample Rate" to 7000
Go to the "instrument" that you want to replace (this is where saving the wav with the "instrument" number can save a lot of headaches).
Click the 16-bit VADPCM Wav button under import

Wait a couple of seconds until the Start and End numbers on the bottom of the window change.
Check the Enable Loop box.

Press the Inject button

Repeat with as many "instruments" as you can (for some reason if you don't replace enough of them, then there will be a "size error" when you finally save it).
When you are finished with all the themes, then you can save the rom (under a different name to be safe).

Open it in the emulator and check the songs in the CAW section to see your work.