What type of speaker do you have? All built in speakers are not the same. We always pack the stud/joist cavity with fiberglass, but it is usually to help stop the sound transfer to another room. The best sounding room for in-ceiling speakers for us is usually a covered porch, where there is open attic and we don't use any insulation behind the speaker.

Make sure your speakers are in phase.

We like to use Sonance in-wall/celing products. They sound good for the use we select, but the real reason is the install is easier than most products. They are ready to go as you open the box and they are very reliable. This is important when you are trying to finish a job and stay on budget.