Since you are a student and your budget must be limited, I would recommend:
A cheap Google Chromecast or a used Sonos Speaker from eBay (max 100€) and use OH2 + https://www.openhab.org/addons/bindings/chromecast/ or https://www.openhab.org/addons/bindings/sonos/
Use your magic knob to control the volume of the speaker (or more)
Use Sonos as an audio sink in OH2 and implement a TTS Service to play with it (example: HowTo: Listen & Talk to your Home)
If you have already covered the Light dimming & blinds aspect, look for additional A/V stuff to control (e.g. the volume of a TV connected to OH2)
How does your “magic” turning knob connect to OH2?
You have some competition already 
Suggestions on Volume Control Knobs for Wall?