I'll never use the lighting dongle and I don't use corded buds/headphones now so I will literally not feel a difference in that respect. I only listen to audiobooks, podcasts, Pandora, and Spotify on the phone, so I'm not exactly getting the greatest audio I could be anyway.
However, having increased dust and water protection are something I'm very happy to get if it improves the longevity of the device. I don't really care too much about it getting that much thinner either, but I'm happy with making the phone more physically robust if they can.