To create a custom SMS tone for a jailbroken iPhone on MacOS from iTunes:

1.) Start iTunes.
2.) Go to Preferences/General.
3.) Click “Import Settings” next to “Ask To Import CD.”
4.) Change “Import Using” from “Mp3 Encoder” or other to “Aiff Encoder.”
5.) Right click on the song you want to convert and go to “Get Info.”
6.) Click on the Options tab and go to “Start Time” and “Stop Time.”
7.) Enter the desired time range that you want and click “Ok.”. It should be less than 30 seconds in duration.
8.) Right click the clip and click “Create AIFF Version.”
9.) Rename the resulting .aif file to sms-received[1-6].cap.
For example: Rename myfile.aif to sms-received1.cap.
10.) Ensure that ssh is installed on your jailborken iPhone.
11.) Replace one of the sms-received[1-6].caf files in /System/Library/Audio/UISounds with your newly created sms-received[1-6].cap file. I used FileZilla to transfer the file from my Mac to the iPhone, but you can use any sftp/scp client or the command line from Terminal.
12.) Now, on your iPhone, you can find your newly create sound in Settings/Sounds/”New Text Message.” The selection names do not change, but you can select each one to hear the sample to find your custom SMS/MMS text sound.
13.) Remember to switch back to Import Settings in iTunes.