I work in an open-plan office environment with approximately 100 other people. It's sort of an unspoken rule (and common courtesy), to wear headphones if you need to listen to anything on your PC, as to not disturb others around you. If you aren't listening to anything, PCs tend to get muted so that there aren't email and Skype notifications pinging every few minutes.
So a new graduate started working with us just over a week ago and they are completely oblivious to the noise that their PC makes. In addition to watching tutorial/training videos aloud, they have their Skype notifications turned on meaning every time somebody comes online/offline; there's a ping.
Another member of my team showed them how to turn Skype notifications off last week, specifically using the term "annoying" to describe it; however, nothing has changed. They seem to be quite a quiet person and I really don't want to come across as being aggressive or rude, but their PC is literally behind me and it is pretty irritating.
How can I ask them to mute their PC or wear headphones without coming across in this way?
edit: Rather than coming up with solutions e.g. wearing noise cancelling headphones - as suggested in similar questions. I'm looking for an appropriate way to approach this person without coming across too overpowering, but in a way that will get my point across.