It's good enough, not great. The Z30 (at the time and kind of still is) one of the best audio experiences of any device that doesn't specifically cater to audiophiles.
The Z30, PlayBook, and all Passport variants have the nicest stereo speaker audio of all BlackBerry's and practically all smartphones.
Aside from devices with specific partnerships for higher end audio that is.
Out of regular smartphone audio though, Z30, PlayBook, and Passport are the best. For me anyway.