How can I reduce the sound of my HVAC condensation drain?
The sound is coming from the fall of the condensate into the water standing in the trap. This is a dishwasher sidearm designed to allow high flow and not obstruct a kitchen drain. Some have a hood in the center that directs high flow down the drain. This hood may be causing the condensate to accelerate in the air down to the standing water. 


You may be able to cut out the center hood inside the tailpiece. This might allow the condensate to drain down the side of tailpiece rather than fall into the trap.  
But if that doesn't work, replace the sidearm tailpiece with a standard tailpiece. Fashion a makeshift sidearm that enters the trap just above the standing water level. You could just drill a hole in the trap and cement in a sidearm just above the level of the standing water. 


In any case change the opaque hose to clear so you can observe the flow of condensate.

Note that if you ever have to use a plunger in this lavatory you should pinch off the flexible condensate line (with a shut-off clamp or locking pliers). See Siphon Hose Shut Off Clamp Small Plastic Clamp Fits 7/16 in O.d.tubing Plastic Tubing Mid-range Clamp. There are other designs for a shut-off clamp, one out of metal with a thumbscrew which actuates a bar which compresses the hose.

EDIT Should you decide to drill a hole into the trap above the standing water level you could possibly use a press fit of tubing into the hole. You'd insert a short piece of rigid tubing inside at the end of clear plastic tubing and force that into the hole. If you use silicone grease around the outside, it will make insertion easier and help seal the hole. Petroleum grease would cause deterioration of the PVC over time. Soap would lubricate, but would not promote sealing, but if you didn't have silicone and wanted to see if the sound was reduced to acceptable levels you could use soap. Candle wax would be OK, I think.

Additional EDIT

You could leave the sidearm tailpiece in place and cap off the sidearm. Then redirect the flow to go into the trap. drill into the trap above the waterline. If you would drill from the side, you'd be able to do this without disconnecting anything. The new transparent condensate drain line could be smaller in diameter and more flexible than the existing one.

More EDITING 

You may be able to fix the sound without any surgery or drilling the trap. In this solution you would use a small enough clear flexible hose that it could be inserted in the sidearm and pushed down so the end of the hose was just above the standing water in the trap. This would enable you to try this idea without modifying the trap or even disconnecting the trap. This would partially obstruct the drain, but it's worth a try.