This will be for very simple sounds for those used to modding AI traffic car sounds in 1.36. More complex sounds may be possible with more experiments. Special thanks to Robinicus and Harven for helping me with the technical side of FMOD.
First download and extract this project file: https://drive.google.com/open?id=10EeIY ... CVjXVQA_rp
This will show you exactly how I've made the sounds for the Subaru WRX STI in AI traffic. Read on for an explanation.
Sound assets
Assets are the sound files used in your FMOD project. You can add your own by going to File → Import Assets
In the Assets tab we have three sound files:
car_tire_noise.wav
This is a tire sound specifically for cars. Feel free to reuse this sound for your own mods, but keep in mind it's not meant for large vehicles like buses or trucks.
subaru_wrx_sti_high1.wav
This is a clean sound loop of a Subaru WRX STI's stock sound while driving. Constant speed sound samples are easy to manage for simple AI traffic car sound modding.
subaru_wrx_sti_idle1.wav
Finally, this is a sound loop of a Subaru WRX STI idling.
Event layout and volume settings
In the subaru_wrx_sti_2015_engine event, you can find how I've organized my sounds. Each sound has a separate audio track.
There are two magnet regions used. One controls the idle sound with a speed transition condition (0.00 to 8.00 km/h).
The other controls the engine sound with a speed transition condition (8.00 to 249 km/h). These transition conditions can be adjusted down the bottom after clicking the magnet region.
Because my tire noise sound loop is not the same length as my engine sound sample, I've repeated the sound and arranged it so it loops seamlessly in the second magnet region.
Here you can also adjust the volume of the sounds using the volume knobs. I have my sound volumes optimized for the medium (default) setting on the traffic volume slider. For sounds of motorbikes you'd definitely want it to sound louder than my example.
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Speed parameter
Now you might be wondering what is the speed transition condition? It is based on the speed parameter. Next to "Timeline" is the "speed" tab. If you press the play button and drag the timeline marker, you can hear how your AI car will sound like at different speeds without having to go in-game.
Note that AI cars have a maximum speed. If you need to increase or decrease the max speed for your project view, right click on the speed tab, select "Edit Parameter" and edit the max range value.
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Pitch curve for engine sound
While you're in the speed tab, expand the "engine" track. This will show the pitch curve I've set up for the engine sound. It's quite similar to the 1.36 implementation. At 8km/h, the sound is at a very low pitch, and as the car increases speed, the curve smoothly brings up the pitch of the sound. One thing I've noticed is bringing the pitch higher than the original sound can cause your sound loop to "finish" quickly, which causes a silence break in the sound. So I don't put the pitch over 0.00 st for that reason.
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This pitch curve, in my opinion, will help make a believable acceleration and deceleration, but it will depend a lot on your sound sample and whether it's in an average and constant speed.
Volume curve for tire noise
Here's something we couldn't do in 1.36.
Expand the tire_noise track to reveal a volume curve I've set up for the tire noise. We all know you shouldn't hear the rush of tire sounds while the car is idling, so we use a volume curve based on the speed of the car. As the car increases in speed, the tire noise becomes more audible until its reached the original volume. While the car is at a very low speed, the tire noise won't be audible at all.
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Volume curve for idle sound
I've also added a volume curve for tire noise, but this is just to help make the transition between the engine sound smoother. Remember with magnet regions, the sound will not play if the speed is outside the speed transition condition. So even if the curve is at -16.0 dB at 10km/h, the sound won't play because it's outside the idle sound speed transition condition (0 to 8 km/h).
Hope this helps!
More complex sounds may be possible, but I think it will require a lot of creativity. Being able to base sound files on speed alone unlocks more possibilities than before 1.37.Read ADVANCED guide
the difference in our sounds is that I will avoid the pitch option for engine sounds and use it only for tire noise. also my sounds will have high and low sounds for cars but I struggle to find a good control on the speed where this transition should happen and of course I want it flexible for each Ai. same as you I will leave the trucks sound at the end hoping also that SCS will update wiki eventually with the traffic sounds too so we can see how they did it 
(in case I decide to make it separately),