I also have the problem that the roku streaming stick+ defaults to mute on power on. It is more than just an annoyance to me. I tend to miss the audio of first few seconds and have to rewind to hear it. I'd like the riloku team to provide workaround and fix.
I have the same problem, but don't have a mute button on my Roku remote. However, my issue is not with a soundbar. Whenever I turn on my tv, the sound is muted and I have to bring up the Roku app on my phone, to unmute the sound. I tried deleting the Roku app from my phone, but that didn't solve the problem. I also looked everywhere in the setting to try and fix the problem with no success.