Rather than use a home audio solution, I would recommend stepping up to a professional system that is more suited to your needs. A good place to visit is any Guitar Center store in your area; they have an entire department devoted to PA systems, including the small portable type that you need.
Here's an example:
http://www.zzounds.com/item--STCAL12R
This is priced at or below the mid-point of most of the systems you will find. There are many that are priced at half this amount, as well as the "sky is the limit" type systems.
This one is a good example of a unit that is designed for maximum portability. There are many variations.
Basically what you are looking for is a system that has speaker(s) and amp with some basic controls for input levels, volume, etc. Critical to your need is a pair of RCA inputs which would allow you to connect any portable CD/MP3/Tape player that has a line out.
Happy shopping, and good luck!