This is just a list of things I've learned so far from my experiences with the Sena 20S (Version 1.5 Firmware)
-Must perform a Factory Reset after every firmware update. See manual for details.
Choppy Music/Intercom/Radio Solution:
-Pop up your antenna! (Sena Support recommends antenna up when unit is being used.)
-SMH5-FM (Firmware 1.2) can host music sharing with 20S (Firmware 1.5), but not the other way around. Sena Support is aware.
-SMH10 and 10C can share with 20S.
Intercom Overlay Sensitivity Levels Explained:
-Not (directly) involved in attenuating music when intercom is active.
-Works when unit is Audio Multitasking (e.g. music+intercom)
-Basically operates as a squelch knob. Level 10 = sensitive = receive "all" sound (weak or strong), level 1 = receive only strong (yelling) sounds.
-Move towards level 10 if first few syllables of intercom conversation are being cut out or if you can't hear your buddies speak.
-Move towards level 1 if you don't want to hear your buddy over the intercom singing along quietly with his/her music.
Music Attenuation during Intercom (whilst your significant other speaketh):
-Turn Navigation App option "on" if you don't need music attenuated when buddy talks.
-Not (directly) involved with Intercom Overlay Sensitivity Levels
4-way Intercom with SMH5-FM:
-Can intercom daisy chain an SMH5-FM onto the end of a 20S line: [20S] - [20S] - [20S] - [SMH5-FM].
-Sena Support says this is not officially supported, but it worked in my case.
-Lots of "popcorn" noises but likely due to improper velcro button mic placement from the SMH5-FM during tests.
-Absolutely! No problems so far.