Actually you can try an alternative method to slow down the speed and maintain the voices not distorted.
To do this you can use a WinAMP 2.x DSP plugin (ex: PaceMaker, Slow Me Down)
I used these plugins some years ago, I hope they are still working.
Copy these plugins in a folder of your choice, and then in BSplayer options (CTRL+P) goto Audio section, WinAmp2 DSP plugins and enable Winamp DSP plugins support and select the folder in which you copied your plugins
