Interesting. For minisphere's Audialis API, I actually abstracted away the distinction--the Mixer object encapsulates both a mixer and voice. The voice/mixer dichotomy confused the hell out of me for the longest time, and until very recently I just stuck to using the default mixer.
The only thing that concerns me if voices and mixers are combined, is that you won't be able to create, e.g. float mixers if the driver in use doesn't support them. This issue actually bit me with the feature mentioned above. DSound doesn't allow float voices, and Allegro balks if you try to create an 8-bit mixer (for good reason!). In order to support these, I had to create the voice and mixer with different parameters. So on reflection, the distinction does have its benefits.