The Roku streaming stick can quite obviously be used to receive Airplay from another source, but can it be used to stream audio via Airplay to another device such as a Sonos sound system? Or from the Roku app to another device?
No, there is no ability to stream the Roku to any other device. Its only output is via the HDMI connection.
There certainly are options to create some sort of broadcaster, but it would be involved, technical, and likely cost a lot, as it would probably take professional level equipment. Most equipment I've seen for sending something from HDMI wirelessly is merely sending the signal to another device that can connect to the HDMI input of a display. That would not make it viewable on a phone, tablet, etc. It simply replaces the long HDMI cable between the source and the display.
No, there is no ability to stream the Roku to any other device. Its only output is via the HDMI connection.
There certainly are options to create some sort of broadcaster, but it would be involved, technical, and likely cost a lot, as it would probably take professional level equipment. Most equipment I've seen for sending something from HDMI wirelessly is merely sending the signal to another device that can connect to the HDMI input of a display. That would not make it viewable on a phone, tablet, etc. It simply replaces the long HDMI cable between the source and the display.