It being short for sound is the one thing that offsets it compared to actual random 3L IO's.
I'm also thinking tokens / crypto. There's SandCoin and SnodeCoin which both use SND shorthand, but these are highly dubious projects.
If any SND token became mainstream, that could make your domain much more valuable. Think 'eth.io' or 'btc.io'.
Unfortunately, this is extremely unlikely to happen.
Other than that, I can see this being used as the project domain for a sound library, or perhaps a small startup / audio software developer.
Maybe mid $xxx of you're patient, but given the reg fee of IO it might not be worth the wait.