A lot of things can effect exhaust sound. Cam timing, rod/stroke ratios, cylinder heads, cr, pipe shape, pipe diameter, pipe length. ect, ect.
Yes, holley pumps are super loud. I hate them with a passion. Get a mallory comp110. They are quiet as a church mouse and can handle a 450hp engine. The mallory comp140 with regulator can handle a 600hp engine.
Flow masters don't flow very well when considering how much noise they make. A OEM like Dynomax Super Turbo out flows a Flow Master and does it at under half the volume. That is the sign of a good muffler (good flow and good sound reduction). Flow masters just came up with a great name for their loud **** mufflers. Hooker-areochamber's have great flow (50% more than flow masters) but are louder than the Dynomax's but still 25% quieter than the flowmasters.