It depends a lot on your budget. Many of the cheaper earphones (Sennheiser CX300's, CX95's, etc) are biased towards strong bass response and start to distort the mids and highs when the volume is increased.
I would recommend a good quality pair of in ear sound isolating earphones, such as the excellent dual driver
Ultimate Ears Super.fi 5 Pro earphones or the customisable
Sleek Audio SA6 earphones.
The Sleek Audio SA6 may be particularly relevant for your wife, as each earphone can be tuned individually to suit each ear. Sleek Audio's Jason Krywko and his father are both audiologists and have been in the hearing aid industry for many years. This is where they came up with the idea of an earphone that can have the frequency response individually tuned to match each of your ears.
Shaun