The V30 has superb sound when either listening through headphones or connecting directly to the car's aux input if you have a good sound system in your car, and it's advantage over the V20 is that it doesn't eat up battery like there's no tomorrow when the HiFi setting is turned on. But ultimately outside of sound quality through the headphone jack, there really is no comparison.
Everything about the S9+ will be miles ahead. The camera, the speed, the beautiful display. If you're really worried about sound, I'd suggest keeping the V20 in the car with a direct connection to the aux port or better yet, if your car audio unit has an SD card reader or a USB reader, your car's head unit probably has a vastly superior DAC to the V20.
I had the V20 and later the V30 primarily for the music. Ultimately I switched to an SD card in my car, and a Note 8 (also trying a Pixel XL 2 in the process). Haven't looked back at that decision.