I agree with what Jason said. The only trouble is that your question almost sounds as though you mean a permanent condition – i.e. you always have to play softly to avoid annoying your neighbors. This doesn’t lend itself to really any muting solution. I mean, any mute you use is going to drastically alter the sound and it’s not a way you’d want to always practice, especially as a beginner, IMHO. Generally these muting techniques are temporary solutions for use when you are say on vacation and staying in a hotel or something like that.
The only other suggestion I have is do NOT play the soprano! Learn on the alto. I’d recommend this anyway for any adult, because the alto is the “main” recorder instrument with the largest repetoir. But also, the soprano is so shrill that it will drive even anyone else in the room crazy, let alone neighbors. I can barely stand to play it myself.
The least annoying recorder would be the tenor (or bass). You could try learning on a tenor (same fingering as soprano) and probably not worry at all about neighbors it sounds so gentle.