The iron that your grass also needs is less available when the soil pH goes much over 6.8, so the grass could have a more yellowish tinge indicating chlorosis due to iron deficiency anemia.

One thing to add is that I believe Sulfur is slow acting and make take a month or 2 to see the results. Also the effects are not permanent, so you may have to apply it every 6 months. Also, the bag of sulphur should tell you how much to add to reach the amount of reduction in soil PH.

Another thing that effects the soil PH is how the lawn is watered. I live in Houston and our water is alkaline (high PH). If we don't get a lot of rain, and we have to use sprinkler water, the PH of the soil will tend to rise. If we get lots of rain, PH is usually pretty good.

Iam in hot Apopka, Florida (north of Orlando). We have not had to water too much this summer due to heavy rains.
Could I add the sulphur to composted manure and apply it to the soil, do you think?

The sulfur will take several months to change the soil pH, that is not something that happens very quickly. Adding that much sulfur to compost would not be a good idea. Apply the sulfur to the soil, wait until after a good rain, or two, and then apply the compost.

My pH is 8 and I don't do anything to change the pH. St Augustine can grow in almost any soil.