Purchase a roll of duct liner..you'll find this product at an HVAC/insulation wholesaler..not the big box stores:
Here's a cut/paste of the duct muffler construction:
Because the basement will have some pretty serious audio, the number one concern is sound traveling through the ducts to other parts of the house, particularly upstairs bedrooms. The strategy has a few layers. First, I'm installing a few duct mufflers, building a few dead vents for supply/return in the basement, and finally lining the main supply and return trunks in the house with 1" acoustic duct liner.
I've tested sound levels at the two upstairs vents, checking them while generating an 84db pink noise signal from my computer speakers in the basement. You can buy duct mufflers, but my situation pretty much requires building them to fit. I designed a strangely similar header and muffler system for my Westfalia van's audio conversion engine, using two different sizes of straight thru stainless mufflers. These are super simple to make, and based on a few online specs, should cut sound levels up to 30 db, in the 1khz range (where voices are centered).
Fantech has some numbers posted for their 2 and 3 foot silencers
For the 5" duct, I'm using an 8" pipe with 8" to 5" reducers at both ends. This should keep flow restriction very low. Step one is to cut the 8" duct material to size. Using a square and scribe makes for a nicer muffler.
I picked up a duct crimping tool to simplify pipe work, particularly for the mufflers.
This mesh will ensure the duct muffler stays at 5" internally.
The simple calculation of D * 3.14 gives you the wide of mesh to cut. I cut it as below to make it easy to join the mesh to form a 5" tube.
Wrapping in 1" duct liner. The liner is duct rated..the smooth side faces in to prevent loose fibers from circulating. I've also sealed the exposed fiber at both ends with duct sealer. This was a mistake cut on the duct liner..I ended up with a full wrap of two layers which keeps the mesh tube nicely centered in the muffler.
Partially assembled. I wrapped the 8" duct material over the core, and wrestled a bit to pop the pipe seam in place.
Duck sealer! This came from Brock White industrial supply locally. It's water based, duct rated, easy to apply (like thickened latex paint) and quick drying.
Sealing the joints.
Sealing both ends to ensure loose fibers cannot enter the air stream.
They're very fast to construct. The larger one is for the 5" duct heading upstairs, and the 4" ID muffler is to manage the supply (hot) air going to the basement bath. The larger muffler is 48" (as large as I could fit in the space) and the smaller one is about 28". If you basically yell into one, the sound level is reduced by at least 50%...surprisingly effetive. I've measure reference noise levels first, so will update as the HVAC sound project is completed.
With basement pink noise (from my PC speakers) generated at 84 db, I'm measuring 47db on the 2nd floor at vent 1, and 35db at vent 2.
Finally to line ducts, pick up some insulation hangars. There are also versions with perforated bases for attaching with screws. The HVAC/insulation supplier should stock these. You can either build up your own dead vents for intake using these hangars to fasten the liner, use mufflers (they're quick and easy to make) where space is limited, and line your air returns if required. I have two air returns which use closed in basement joists as ducts (very common), so lining these will make a difference. It also cuts air flow, which I addressed by adding a larger air return in the basement via a vertical dead vent. Note that duct liner has a side that faces air flow. Any exposed ends you can coat with duck sealer (or another plenum rated sealant) to prevent loose fibers flowing about.
Hope that helps. A post like this would have saved me a few days of reading and searching :-)