The arts-1.3.0 currenty has a "jack"-USE-flag dependency on media-sound/jack. IMHO the correct package must be media-sound/jack-audio-connection-kit. This may result in arts lacking jack support (if jack-audio-connection-kit is not yet installed), but at least pulls in an additional package (media-sound/jack) and its dependencies that is probably useless an unwanted for most people ;-) Reproducible: Always Steps to Reproduce:
Fixed - thanks for the report.