Usually streamer use youtube for music in theyr stream, but they configure that audio and volume. If twitch can send 2 audio channels, 1 for the streamer with voice, and 1 for music, where streamer choose playlist, and music volume can be set by the viewer (for himself). It should work with a music program, example integrated with spotify, so every user can listen streamer playlist or personal, and set volume alone for himself
This forum is for third party developers that integrate with the Twitch platform. For site suggestions, I would recommend filing at https://twitch.uservoice.com/.