Ok, I got it!
For all interested in the entire code to get Binding and Rule logs into one log file:
# MyLogger - Sonos (Rules)
log4j2.logger.Sonos_Rules.name = org.eclipse.smarthome.model.script.Sonos
log4j2.logger.Sonos_Rules.level = INFO
log4j2.logger.Sonos_Rules.additivity = false
log4j2.logger.Sonos_Rules.appenderRefs = Sonos
log4j2.logger.Sonos_Rules.appenderRef.Sonos.ref = SONOS
# MyLogger - Sonos (Binding)
log4j2.logger.Sonos_Binding.name = org.eclipse.smarthome.binding.sonos
log4j2.logger.Sonos_Binding.level = INFO
log4j2.logger.Sonos_Binding.additivity = false
log4j2.logger.Sonos_Binding.appenderRefs = Sonos
log4j2.logger.Sonos_Binding.appenderRef.Sonos.ref = SONOS
# My File appender - Sonos.log
log4j2.appender.Sonos.name = SONOS
log4j2.appender.Sonos.type = RollingRandomAccessFile
log4j2.appender.Sonos.fileName = /var/log/openhab2/Sonos.log
log4j2.appender.Sonos.filePattern = /var/log/openhab2/Sonos.log.%i
log4j2.appender.Sonos.immediateFlush = true
log4j2.appender.Sonos.append = true
log4j2.appender.Sonos.layout.type = PatternLayout
log4j2.appender.Sonos.layout.pattern = %d{dd-MMM-yyyy HH:mm:ss.SSS} [%-5.5p] [%-50.50c] - %m%n
log4j2.appender.Sonos.policies.type = Policies
log4j2.appender.Sonos.policies.size.type = SizeBasedTriggeringPolicy
log4j2.appender.Sonos.policies.size.size = 10MB
log4j2.appender.Sonos.strategy.type = DefaultRolloverStrategy
log4j2.appender.Sonos.strategy.max = 10
Once again, thank you very much and have a nice evening!
Edit: Changed DEBUG into INFO.