Hey mate, try to do the following (it worked for me):
0) Copy your sound file to \m-r-software\Omsi\Vehicles\MAN_SD202\Sound\ (I'm using SD202 D92 for this example)
1) Open keyboard.cfg located at \m-r-software\Omsi\Inputs
2) Add an entry with a command name like that:
* I chose 'key_mycustomsound' as the name of the keyboard event.
3) Open sound_D92.cfg located at \m-r-software\Omsi\Vehicles\MAN_SD202\Sound\ (Again, I'm using SD202 D92 for this example)
4) Add an entry as shown below:
Please note that here we have a 3d entry. If you want, you can change this coordinates to set where in 3D-space this sound will "play", relative to your bus model. In this case, these are the coordinates for the bus cockpit. That 0.5 under yoursoundfile.wav is the volume, I presume. The name ev_mycustomsound is a name I gave to our EVENT.
5) Now we need to insert some lines to the script. Open the Script folder. I'm going to use the script Cockpit.osc. Open it on Notepad.
6) We will assign the TRIGGER key_mycustomsound to the EVENT ev_mycustomsound, which will be triggered. I prefer to add a line at the begining of the script.
Add an entry like this:
T.L.ev_mycustomsound is loading the EVENT ev_mycustomsound, which has been linked to our sound file in Sound_D92.cfg
7) Save these files, open OMSI and go to the Keyboard (or Joystick) settings and search for our new created event. Assign it to a key of your taste, open D92 and make a test. It worked here for me. I don't know how to do this in less steps, but what matters is that works
Obviously this can be improved, for example, to only make the sound play when the bus electrical supply is on. OMSI scripting universe is great!
Hope I have helped you.