Hello,
The Piko decoders used to be Uhlenbrock's G-Scale decoder (77500, 3A now 5A), but are now provided by Massoth. The plug-in sound units are provided by Dietz (they used to be the MicroXS units, but they have now been replaced by the MicroX3 model). The sound units are also available from Uhlenbrock under their 'IntelliSound' label.
The sound quality of Dietz sound decoders is excellent and generally (together with Zimo) considered to be the 'gold standard' in terms of sound reproduction - I have a few myself. I cannot comment on the Uhlenbrock decoder, as I have never bought one.
Marc