I have always had problems with various devices used as a music on hold source. Sometimes you gan get a CD player or radio sounding OK and other times you have to struggle with the sound.
There are a lot of things going on. For one the tablet is expecting headphones with a lower impedance then the 600 ohm MOH inputs on the PBX
Here is another older thread talking about a similar issue with a Diskman:https://sundance-communications.com/forum/ubbthreads.php/topics/124440/Low_Volume_Music_on_Hold
Sometimes I have had better luck using a stereo 3.5 mm plug with a Y adapter to a mono RCA plug rather then using a mono mini-phono plug.
The MOH volume can be adjusted using programming software not with a dial on the processor, but I don't think you could amplify the source enough to make is sound good.