And the simulators are not fully qualified for music playback.
Maybe, some day, it will be possible to have Rockbox as a standalone player application for x86 and x64 platforms. We're already developing Rockbox as an Application for Android and more recently Maemo, so perhaps one day Rockbox can run as a software player. It could have some advantages over other players, like supporting many file formats with low resource consumption (assuming the codecs would be so optimized as they are, for example, on ARM), and being fully themeable.
Anyways, IF anything like this gets implemented, it won't be on a near future. In the meantime, on the PC you can use a player like VLC which supports many sound and video formats, and even plays damaged/strange files.