Put a speaker entity inside it.

@Chippoka

Yes, I considered this option, but there are also other problems arising from it:

1. The funnel muffles extraneous sounds, so even ambient_generic on volume 10 will sound quieter than usual.

2. When moving the funnel, the sound will not be played, because pre-set triggers do not move with the funnel.

On 2., place the entire funnel travel region in the sound trigger (there's a way to play a sound throughout the area while in the trigger space). On point 1.? Not sure, maybe there is a way to modify the sound associated with it or called by it (prop or script); look in the script folder to see if there might be something helpful there. Its been a long time since if fiddled with Hammer. Wish I could have been more helpful.

Regarding the funnel, I meant moving it using the portal, so that all the triggers remain on the other side of the portal and the sound is not played.
I think I should just do a better job of searching online. However, you still helped me a lot in resolving this issue.