It’s possessive, but represents a grey zone in how apostrophes work in modern English.
A trailing apostrophe is for a possessive following a plural “s”. But “Sonos” is just a made up name, meant to evoke the Latin “sonus” I’d guess. Regardless, it’s a singular name, the fact that it has a trailing “s” doesn’t mean it’s plural.
So in this argument, a possessive should be spelled “Sonos’s”.
BUT! There’s a longstanding (but eroding) tradition that there’s a specific exception for classical or biblical names that end in an s. Is Sonos a classical or biblical name? I don’t think so, but I suppose people could decide it ought to be treated like one.
Finally, recent advice on this is to let pronunciation guide the presence of a final “s”. So if you pronounce Sonos’ as “soh-nohs” and not “soh-noh-ses”, the APA thinks Sonos’ is reasonable [0].
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