'Walkman' is a complex word that means something entirely different than its constituent words, i.e. it does not refer to a man who is walking. In this case, 'man' doesn't carry its original meaning any more, but it simply a part of a larger word, whose plural should therefore end with a regular suffix '-s'.
A computer mouse, even if it isn't a real live walking cheese-gnawing animal, is still two separate words, one of which ('mouse') denotes something we unanimously call a mouse. Since it is a very much individual word, it will follow its irregular pattern and become mice when we want to talk about 'computer mice'.
Sony evades the issue by talking about 'Walkman Personal Stereos', and some companies do the same thing by referring to 'mouse devices' instead of mice (e.g. Microsoft, on the right-hand side of [url="http://www.microsoft.com/products/info/default.aspx?view=22&pcid=c250fb0a-1613-4550-983d-ba203f35... "]this[/url] page). Other companies however (e.g. MicroDirect, on the product headers on [url="http://www.microdirect.co.uk/catalog/mainprodlist.asp?txtProdGroup=Y&txtDesc=MOU"]this[/url] page) choose the easier way and just call them what they are - mice.
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That's not neccessarily correct. The plural of the name of the hardware used to move the pointer on a computer screen is not mice. No one, not even the inventor of the mouse, agrees on the proper terminology for the plural name of this device. Thus, many companies, such as Microsoft, use the term "Mouse devices", not "Mice".