If you would like to play music in the background while you stream with your mic on, you can enable it from your computer's built-in audio mixer.
You will have to confirm whether or not your computer has a built-in mixer on your own.
However, a built-in audio mixer is usually included in Windows.
You can check to see if it's on your computer by doing the following:
1. Right click the speaker icon on your task bar. Select "Open Sound settings"
2. Open the "Sound Control Panel" and make sure disabled devices are visible
If you're using the Windows 10 Volume mixer
1. Right click the speaker icon on your task bar. Click "Open Sound settings"
2. From the Sound settings, click "Manage sound devices"
3. Check to see if "Stereo Mix" is disabled or not. If it's disabled, enable it.
4. To adjust the volume, right click the speaker icon on your task bar and click "Open Volume mixer"
Please note, however, that copyrighted music is currently prohibited on pixiv Sketch LIVE.
You can only use royalty-free music or your own creations.