A couple of things you could think about:-
LME49725 is very close to LME49720 performance-wise, but has a typical quiescent current of 6mA as opposed to 10mA, if you can get them. Unfortunately Farnell have discontinued them, but it's still an active device.
You could sign up at ti.com and ask for some samples. Free samples. TI have bought Nat Semi but both did free samples. TI might not have got their act together with the National parts yet, but no harm in trying.
OTOH, NE5532s are almost certainly good enough and you can get 20 for $4.50 shipped from ebay.
Unless you're determined to put the charger in the unit, you could use tubular Lithium cells with an external charger (no design issues), they have better energy density than NiMH and you need fewer to get a decent voltage. Ultrafire protected CR123As from dealextreme.com are pretty good.
A graphic equaliser or at least parametric tone controls might be more useful than simple bass and treble for somebody with hearing problems. A graphic built with preset pots and SMT components will still probably fit in an Altoids tin. It's a set and forget application.
w