A game developer named Peter Madsen has started work on an extension for Twitch called SpotifySynchronizer. The idea is pretty simple. It allows viewers to synchronize their Spotify accounts with the streamer – so they can hear the music on their own account. Viewers can hear the same music at the same time as the streamer, while music artists, Spotify, and PROs get paid. Viewers who are watching without the extension will only hear the standard game audio.
Source: Twitch Streamers Use Spotify Syncing Extension to Avoid DMCA Strikes