I've heard a lot of other people say that certain foods make their tinnitus better or worse. I must admit, I have not found any links in my case, but I still believe there is a mechanical component to my hearing problems, so there wouldn't be.
The few studies that do exist have not found very strong relationships between diet and tinnitus, if any, but we are all subtly different, so it is possible.
Perhaps it was the caffeine in the coke? Again, this has no effect on me, but it is a stimulant and can therefore heighten awareness.
Unlikely to be relevant as you are asking about sugar, but phenylalanine is known to affect brain chemistry and this is present in diet coke - and lots of other things besides.