Never thought the subject of earphones would come up for discussion here (!!) but I use them all the time as a live performing musician. They are, without a doubt, the most important discovery I've ever made in my job as a gigging musician and offer the greatest protection possible on a loud stage environment. For those of you that haven't stood on a stage, next to a rock drummer in full assault mode and a Marshall Stack screaming at you, you would not believe how loud it can get. A drum kit, played properly, is an incredibly loud instrument, the cymbals alone can make you wince and wish you weren't there... In-ear earphones, fitted with moulded earpieces or foam buds, cancel out all the noise and allow me to hear the mix of the band at a volume I'm comfortable with, which is just wonderful compared to the volume I used to have to tolerate and is why you hardly ever see a live performer without them these days.
The best you'll ever find are made by an American company called Ultimate Ears, and the model is called the Triple-Fi 10. These are, in my experience, the finest hifi earphones in existence. As for hearing conversation, that comes down to the type of material used for the buds which are the squashy bits that sit in your ear and provide the 'seal' from the outside world. I use a bud called 'Comply Foam' which heat up with your body temperature and provide a perfect seal which also enhances the sound quality as the bass reproduction is wholly dependent on 100% contact with your skin. Any gaps would ruin the bass. This perfect seal however makes it almost impossible to hear anything that isn't coming out of the in-ear speakers, i.e. conversation next to you. When I have music in mine you could scream at me less than a few inches away from my face and I wouldn't be able to hear a thing from you.
There are ear plugs/defenders that do cut out general noise but allow close proximity voices to get through and there are some earphones that do the same but to get a really nice sound from earphones you need a perfect seal... but with an engine like Eric's who wants music anyway?!
Andrew.