Basically, mp3 are simply xored with 0xC5, mp4 are also xored with one byte (every 5 files has different byte), all files has additional 4 byte header and png are not actually png, but pvr files in most cases, encrypted with something simple and also lz4 compressed. Decryption tool works more or less fine - for example, it can decrypt, but can't convert some pvr files, though they can be previewed and exported in any compatible tool.
You can request AES keys on rin forums
(the list with keys is also there)
AES keys finder and latest UE4 bms scripts: in this post