Do you handle music class with low and severe functioning kids, nonverbal and with very limited physical ability?
What kinds of lessons do you have for them? Can you use picture symbols when you want to communicate with them? Do you have simple instruments? Most students can play instruments with hand over hand assistance. Have you also asked their teacher for assistance?
Any time that you can marry action to the use of the instruments you will be more successful. Case in point would be marching while playing, possibly creating audio files that have blank spaces for students to contribute with their own use of instruments. Cuing students with the sound of a bell, triangle, or drum sticks and then encouraging the little boy with the bell to play, followed by the next student cued to use drum sticks, etc., can marry sound to instrument name, image, etc. Some students may not be good candidates for some of these instruments because they can't be trusted not to hit others, etc. You can also use clapping, stomping of feet, and other variations, keying in on beat, counting the beats, marching to the music, and the list goes on. Recorders, if available, may or may not be appropriate. And never forget that the voice is a musical instrument, even when non-verbal. You can also add body movements like rocking in their chairs, foot movements from a seated position, as well as hands and arms above the head. Be creative. Many students who are not overly demonstrative when listening to a tape recorder become engaged when they can feel the vibrations of many instruments actually being played, where they can place hands on the instrument. Let me wish you the best of luck.
There's read-alongs like "Abiyoyo" where certain words cue instruments. That might be enjoyable for them and it's a very basic way of teaching leitmotif. You can possibly use props like scarves with straps to touch on any "respond to music"- type standards. If they're capable of indicating yes/no or between two choices, you can do teaching musical opposites with tempo, dynamics, etc. Listening maps may be another accessible tool.