You could use bluetooth on the ST300 to send music from your phone etc, or upgrade to the SB700/500 to use the built in streaming options using the Bose music app or alexa and sometime in 2019 google assistant and Airplay 2.
FYI - ARC issues also affecting the following device as well.
Things were working well for the past 7 or so months until a recent update either to the TV or the Soundbar. I suspect the sound bar since ARC audio stopped working immediatly after the bose was updated.
[LG OLED65B7A, Software Version 5.80.15]
[Bose Soundtouch 300 Software Version 20.0.6]
Hope a new update comes out soon to fix.
Thanks for posting!
The best recommendation here is to stream your music to the soundbar via Bluetooth so you won't have to remove the ARC connection every time.
Tony A - Community Support