I'm still using a number of Motorola Defy phones from 2010-2011 which are IP67+ rated (30 minutes at 1.5m water depth). Needless to say they have 3.5mm headphone as well as micro-USB connectors. The solution to this problem lies in a simple rubber plug which closes the hole when the connector is not in use.
Does it work? Well, one of the phones used to be my daughter's. She forgot to take it out of her pocket when she put her trousers in the washing machine so it ended up going through a full wash and spin cycle. It still worked, there was no water ingress in the phone itself. The earpiece did seem to have gotten damaged so I replaced it (5 new earpieces for $2.50 incl. shipping, I still have 4 left...). This phone is now used as a media player, running mpd which can be remotely controlled from other phones in the network. It connects to my jobsite radio (an oversized and overweight boombox I made around an old car MP3/CD/Radio with 2x40W speakers with bass reflex and a SLA starting helper/power pack) using that 3.5mm jack.
Does it work? Well, one of the phones used to be my daughter's. She forgot to take it out of her pocket when she put her trousers in the washing machine so it ended up going through a full wash and spin cycle. It still worked, there was no water ingress in the phone itself. The earpiece did seem to have gotten damaged so I replaced it (5 new earpieces for $2.50 incl. shipping, I still have 4 left...). This phone is now used as a media player, running mpd which can be remotely controlled from other phones in the network. It connects to my jobsite radio (an oversized and overweight boombox I made around an old car MP3/CD/Radio with 2x40W speakers with bass reflex and a SLA starting helper/power pack) using that 3.5mm jack.