That doesn't make sense. Once you connect it to the rest of the network, it's no longer "air gapped".
The phrase "air gapped" doesn't signify something special about air, it refers specifically towards breaking the connection (which is typically electrical) between that system and the rest of the network. Perhaps it is not the best term, and "completely isolated" would be better.
If you set up an IR LAN, or use sound, or whatever, then the system is no longer air gapped and you have created a potential vector for information leakage and potential penetration. Sure, probably nobody is going to bother, if the implementation is unique and nothing you have on that system is of particular value, but there have been a number of high-profile compromises of "air gapped" systems and networks (e.g. Iranian nuclear production facilities), that show it can be done even without an intentional connection if the desire is really there.
There are scenarios where partially isolated systems can offer a real benefit, though. I have periodically seen ideas for logging systems that use a 100BT (not Gigabit) Ethernet connection with the Tx pair cut, so that traffic can only ever go INTO the system and never back out again. The system sits on the far side of this one-way hardware gate, listening and logging, and is extremely difficult (although not impossible) to compromise because of the lack of feedback. Note if you want to do this, you need to use old 10 or 100BT network cards that don't have GigE capability, because I believe GigE uses all the pairs in the Ethernet cable in unpredictable ways; you don't have the old Tx pairs / Rx pairs / shield pairs like you used to be able to count on (and selectively cut). I think you'd need to make sure the cards didn't support auto MDIX as well.