I have a 2018 Mazda3 and it annoys the piss out of me that the album art randomly works for wav files. (Only if by some miracle the album is in the lousy Gracenote database). The engineers who wrote the program are the lowest of the low. Their code refuses to look in the folders for album art if the files aren’t mp3s. What century is this? We should be using lossless files.
The crap software also refuses to acknowledge that an artist may not be its database, so instead of using the names provided by the folder names, it tries to find a best match. Worthless!
You shouldn’t use .wav in any century if lossless is your goal. But unfortunately it looks like Mazda added .flac support in 2019. .alac works fine on an iPod Classic FWIW.
Gracenote databases need to be updated. Also installing AA/CP might be useful, it’s fairly easy to do by yourself, I did it in 2018 CX-5.
I am either playing over Bluetooth or from a CD. I haven’t tried a USB stick, though if that’s the solution I probably won’t bother. I have a USB port below the stereo, but if I plug my phone into that there doesn’t seem to be any option for me to play music from it , so I assume it only works with a usb stick.
You might see if there’s a firmware update for your audio system, if you’re comfortable attempting.
Check if there’s USB ports somewhere else, especially inside your center console. Often only one is dedicated to music, I am not sure if this is the case in yours. If by “below the stereo” it means it’s obviously part of the stereo system and not in the tray below, then I’d think that was an audio port and something else is needed.
With a 32GB USB stick, wav is just fine for lossless; the spacial savings for flac aren’t worth the effort.