This requires a major re-write of the entire music player. Given the extensive content already available on youtube, I unfortunately can't justify the amount of work needed for this vs. the value add from soundcloud integration.
Will possibly revisit later again when we have more development bandwidth.
I think now may be the time to rethink about this.
With Twitch using YouTube's content ID system, using YouTube videos for music now makes it harder for the broadcaster to create highlights. If the ID system notices copyrighted material the highlight/archived content becomes muted for 3 hours.
Because of this, Soundcloud is becoming a much more popular option for broadcasters. I strongly encourage you to integrate Soundcloud support into your bot since the usage of Soundcloud will continue to increase.