By default the sounds only appear to work for the Kenworth trucks.

To get them working for the Peterbilt trucks...

Find the following path in your Steam installation folder:

...\steamapps\workshop\content\270880\646829078\universal\sound\truck

In the "truck" folder you'll see 3 folders:

common
default
kenworth_w900

Make two copies of the "kenworth_w900" folder into the same "truck" folder (copy it and paste it twice, if you're using Windows), making 2 duplicates.

Then rename the two duplicate folders to match the names of the Peterbilt trucks in the same format as the kenworth truck. So rename the first duplicate folder "peterbilt_579" and the second "peterbilt_389"

So now your truck folder will have the following folders:

common
default
kenworth_w900
peterbilt_579
peterbilt_389

Start (or restart the game if it was running when you did this) and try it again.