Morally, yes. According to Roblox, maybe? If you re-upload the asset, there’s a chance Roblox may just delete it, claiming it to be copyrighted(happened to me when I uploaded a rain sound effect already on the platform, so I think it could happen in this situation too)
Interesting scenario where it’s someone’s original work and they’ve not expressed any claim to copyright or usage. It’s not exactly creative commons without the artist stating so, but if they’re unreachable it seems (in my opinion) to be in the same gray-zone as abandonware.
Will you get moderated? Well, I really doubt the auto-moderator is going to get any hits when trying to find it online… you could be get a DMCA takedown request if the artist or some entity claiming to own the copyright ever shows back up with proof it’s theirs. At an extreme stretch, whatever midi platform this was made in the artist may have purchased/licensed instruments to use in producing it… but that would be an incredible edge-case to ever present legal problems.