In Linux you can use Audacity: record after setting correctly in Pulse Audio Volume control -> beginning with monitor ….
See for details this tutorial.
speech to text in Linux con KDE connect
If you cannot speak-to-text directly from the smartphone to PC, you can before dictate a text within KDE Connect and then, after, send it to your PC (provided that your smartphone and your PC are both connected with KDE Connect):
KDE Connect (on smartphone) – > remote input -> small icon on high, as below
Then choose as keyboard, Google Voice Typing
Then you will be able to dictate your text (seeing it, before, on the smartphone [KDE connect] screen) and send it to your Desktop device.
