Pinball Fantasies comes pre-configured with Sound Blaster enabled. It doesn't sound that great, because PF's own software mixer plays the MOD channels separated on L/R and adds a lot of aliasing artefacts (also the L/R channels are swapped, but that's not really an issue).

Fortunately both DOSbox and the game support Gravis Ultrasound (GUS) and Pinball Fantasies doesn't need the GUS MIDI patches to work. Enabling GUS in the .conf and in the launcher works out of the box, the mixing is right (because it's done by DOSbox now) and the aliasing is gone.

But once I launch a table, the sound slows down. Does anyone know a way to fix this?

Try increasing the amount of cycles (Ctrl-F12).
If that doesn't help you could try lowering the soundbuffers, for some reason GOG has configured the buffers twice the amount of the default Dosbox setting.