I used to brew coffee soda using yeast. Sometimes it was REALLY good, some batches would be on the bitter side. Basic ingredients were strong brewed coffee, vanilla beans, raw sugar and a light ale yeast. I kept it at a less sweet side and once mixed, bottled it up in those heavy brown fliptop bottles. Only thing was when it sat longer in the bottle, opening them was like opening a bottle of champaign. Really thick foamy head. It was pretty cool.
mmm... I think I may need to make a batch. It's been years.
I was thinking about getting a soda stream machine and just carbonating strong coffee. REAL coffee soda is even a better idea.