I just tested with the "Fourier Pitch/Tempo Control" Winamp DSP plugin and it does work!!
If you installed the plugin correctly, rightclick on the control panel to get BS.Player's context menu.
If you then select "Audio" the "Fourier Pitch/Tempo Control"-option should be shown under "DSP-Plugin".
Please do take note that a media-file must be loaded, otherwise this option will be "greyed out"!
Select the plugin by clicking on it so that its window pops up where you can change settings by moving the slider.
For 1/2-speed playback I selected 185% and sound was quite normal.
I don't know what playback-speed you selected, but you can try playing with the sliders settings.