I'm a long time lurker but first time poster here. There are 3 ways to stop the passengers complaining:
1. Drive properly. However, some maps like Smarden require a lot of stopping, starting and harsh acceleration; so this probably won't work.
2. Adjust the sound settings as per the posts above. This is really just a quick by-pass though, the passengers will still complain, but you won't hear them. They might still storm off the bus at some point, and other sounds that you might want to hear will be off.
3. The actual method to do this properly is to adjust the {whinge_prop} setting in the ticket pack that a map uses. The ticket packs can be found in your 'TicketPacks' folder in your OMSI installation, and the setting is in the otp file in each folder here. You can open this with a text editor (such as notepad). You'll see an entry quite early on such as:
[ticketpack]
0.2
0.8
0.0
0.0
It will be the bottom figure you'll want to adjust, the closer to zero it is, the less likely the passengers will be to complain. Set it at zero, and they shouldn't complain, even if you go around like a rally driver after a few glasses of scotch.
If you need to find out what ticketpack each map uses, check the global.cfg file for each map that you use (this will be in the maps folder of your OMSI installation). It is a bit of a longer process, but then shouldn't throw up any errors with missing sounds.