It is possible that another switch is already set as a MOH source. You can only have 1 source per site, although it should still let you check the MOH source check box.
It sounds like you have access to the Shoreware Director. I hope that you are using only Internet Explorer and not any other browser. That version of Shoretel should support IE version 11.
If you look on the Primary Platform hardware and scroll to the right, the summary should show you which Shoretel switch is setup for the jack based MOH source. In my testing I was able to select multiple sources. I would make sure only one is set as the source. In most cases the other default settings should be OK, but just make sure you set the default source back to Jack Based. The internal calls doesn't really matter.
There are 2 jacks on the Shoregear switch, one for paging and the other as the music source. The MOH jack should be able to connect using either a mono or stereo plug. Here is a bit more information about the plug.https://oneview.mitel.com/s/article...ng-On-The-ShoreGear-Switch-1387479455398
(should not require a login to view)
The video linked above describes setting up a file based MOH source. Unless the customer has a specific need for a MOH player, this is what I use. The 2 issues I have are that the music is usually a bit loud and the file always starts at the beginning of the file, rather then at a random part of the music.