I doubt the mufflers will help. The issue is the "high velocity" which creates turbulence which creates noise. About all you can do is decrease the velocity by using larger ducts and larger diffusors OR more (meaning lots more) small ones.
Now to my soap box ....
I have a potential client that is very focused on specifying duct size, diffusor size, etc. Here's how it works when you're dealing with other trades (such as HVAC contractors). If your objective is to have a duct of size "x", then specify "x" if that's what is important to you. I'd suggest, that duct size is of no importance to you in an acoustic application. What is important is noise levels, etc. So if the noise level of the air leaving a diffusor is important to you then specify (as an example), the Noise Rating (or Noise Criteria) shall be not more than 21 when measured .5 meter from any diffusor.
Another example ... are you concerned that the HVAC will provide 18,000 BTU's of cooling. I'd suggest that's not what you're really interested in. Here's a better way: "The system shall maintain 70 degrees with an outside temperature range from -30 degrees to 110 degrees, humidity shall not be less than 25% nor greater than 35%...."
Let me explain one (not all) of the reasons for this. If you tell your HVAC contractor that he must meet a given temperature, humidity and NR, then if you don't get that, it is his problem to fix, at his expense, at his potential loss of license. On the other hand, if you tell him you want 10" dia. ducts, and he gives you 10" dia ducts but the air noise is outrageous and the room will get no cooler than 85, it's your problem, not his.
Think about it.