Playlist (mp3) won't play on new car receiver

Years ago I copied my 250 CDs to my Mac. I created a 1500 song playlist (mp3) and uploaded to my new (2012) auto hard disk. Been playing great. I have an older RV whose radio/CD player conked out and I bought a new Sony receiver/Bluetooth/Aux & USB input. I copied the playlist to a flash drive for the new Sony. The Sony apparently reads the flash drive, then says no files found. The box says USB accepts MP3, WMA, AAC, and FLAC. I called Sony and they said try MP4.

First question, does this make sense since the box advertises the receiver accepts MP3?

Second question, if copying all 1500 songs to MP4 format is THE answer, is there any way to do a bulk transfer of the many songs?

Third, is there a different, better solution?

I am a real novice when it comes to Music - so if I've missed something easy and obvious, please excuse my ignorance.

Thanks in advance for any help.