Does "(You)" still display on the links replying to you? It's just the sound that doesn't play? You didn't disable it in settings?
Maybe your browser just failed to download the sound file. Can you check that both of these links can load in your iphone's browser? (I think the first one won't actually be able to play on the iphone, but as long as at least one of them works you should be good.)https://mlpchan.net/static/notify.ogghttps://mlpchan.net/static/notify.mp3
After making sure the links can load, see if the reply notifier sound works in the site again.
I'll test myself with an iphone eventually.