The expression language operates only on values from individual files. There's no way to compute something like an average of values from a set of files.
The only workaround I can think of would be to manually set and periodically update album ratings. Create an album-relational field (say [a.Rating]) to record an album rating. Use or create a Categories View displaying [Album] as the category (i.e., in the top pane) in a Details list style. Add [Rating] to the view. This will list all albums, showing the average value of [Rating] for each. Sort this by [Rating], and use that to select all albums for which you would like to set a particular [a.Rating] value. If you need a more precise value than whole stars, use an expression column to display [Rating]—that will show the actual numerical average. When creating [a.Rating], you may prefer to make it a decimal type, rather than an integer/star field.
This seems cumbersome, but it would only take a few minutes to set appropriate [a.Rating] values for your entire collection. The same thing could be done with [Number Plays] (i.e., create an album-relational field [a.Plays]), and the two combined in some fashion using an expression.