We had dinner at Poly (Ohana) last week, and had a seat by the window with a great view of fireworks. They also dim the lights and pipe in the music. At the beach there is apparently music also, but the speakers are so quiet you can hardly hear it I am told.
We enjoyed seeing the fireworks in the sky while eating dinner. However that said, you are not that close - seven seas lagoon is very big!!
So my advice is not to go to these expensive dinner location just for the fireworks experience alone, unless you really also want to eat there too, as the view is good, but not incredible - however if you are planning a dinner at these locations anyway, then sure its nice to get an ADR around time of fireworks. We had a 8:15 reservation, so we were in middle of main meal when Wishes began. You can get up though and get a better view if you do not have a window table - but that sort of defeats the object in my opinion, as you may as well be stood outside!
Finally comment - I did think that more would have been made of the lead up to the fireworks begining, but it just sort of happened - lights lowered slightly and music began - I guess if you were sat in a different part of Ohana, and didn't know it was happening, you may think they were just changing the mood lighting, and miss the fireworks completely.