[BSPeter]

If you didn't change the default key definitions:
[F6] =decrease playback rate by 1%
[Shift]+[F6] = decrease playback rate by 10%
(Repeated depressing of these keys leads to an accumulative result.)
Alternatively:
rightclick > Playback > Playback rate > 1/2x (or decrease by 1%/10%)

P.S.: Maybe combine with Fourier Pitch/tempo Control-Winamp2-plugin to control the pitch of the sound.

Originally Posted by BSPeter

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.

Originally Posted by BSPeter

For users which maybe are unfamiliar with the use of WinAmp2 DSP plugins for BS.Player:
Please go to:
(rightclick >) Options > Preferences > Audio > Winamp2 DSP plugins
There you can select if Winamp plugins should be enabled/supported.
Also (in the south-east quarter) you can select the directory/folder where you saved your WinAmp2 plugins.
Finally you can select the plugin to be used and (with an appropriate media-file loaded in BS.Player!) you can Load it and access and set the plugin's properties, using the button "Config".
Further ..
while playing a file, you can also access the active DSP plugin and set its properties through rightclick > Audio > DSP plugin > etc.
Enjoy!