When working with m4a/mp4 files, rating is always nil in SFBAudioEngine.
I checked the MP4 mapping in SFBAudio​Metadata​+​Tag​Lib​MP4​Tag​.mm: there is no read/write mapping for rating. As a result, ratings set by external tag editors are not visible, and any tag refresh can clear ratings.

Expected
• rating should be read from MP4/M4A (at least rtng or ----:com​.apple​.i​Tunes​:​POPULARIMETER)
• rating should be written back to MP4/M4A

Actual
• rating == nil for MP4/M4A even when rating is set by external editors

Location
Sources​/​CSFBAudio​Engine​/​Metadata​/​SFBAudio​Metadata​+​Tag​Lib​MP4​Tag​.mm

Optional
Many editors store rating in:
----:com​.apple​.i​Tunes​:​POPULARIMETER (email\0 ratingByte playcount)
So adding read/write support for POPULARIMETER would help with compatibility.