Your throat is numb, and you can hardly swallow saliva.

This sounds more natural.

It is as if you were stiflinged after being sprayed with that lidocaine.

“Suffocated/smothered” could also be used in place of “stifled.”

You close your eyes while these people insert a black rubber tube through the mouthguard into your mouth and throat.

I deleted “these” since the people have not previously been specified.

As time passes, you lapse into a half-trance of your own, inhaling and exhaling, the mouthguard clenched between your teeth, the tube making regular motions through your oesophagus and stomach.

Your arms lie languorously hand in hand on the couch.

“Limp” would be the more common word for “languorously”

InWith this distorted, lethargic vision, it feels like you are playing thea clarinet that has lost its form and theninstead has been inverted into the depth of your body.

“Vision” is typically something that is had, not something that is a state of being so I changed “in” to “with.” I changed “then” to “instead” because I think it flows better. I dont normally hear “then has been” if you wanted to use “then” I think it would have to change to something like “then was inverted.” I think because “then” implies an order of past events the tense has to change.

You haveare wearing a mouthguard between your teeth.

When we say between your teeth we're usually referring to the interstitial space (ie. the gaps) between your individual teeth.

It is as if you were stiflinged after beinghaving been sprayed with that lidocaine.

As time passes, you lapse into a half-trance of your own, inhaling and exhaling, the mouthguard clenched between your teeth, the tube making regular motions through your oesophagus and stomach.