The Sonos speaker will support "multiple voice platforms and music services," but the filing didn't specify which assistants and services that might be. Sonos has recently been gearing up for a wide integration with Amazon Echo, so Alexa could be a possibility. Sonos products are sold at Apple's retail and online stores, but that's not exactly an indication that a new Sonos voice-enabled speaker would include Siri support, especially since such a high-end music speaker would be a direct competitor to HomePod come December.
Zatz Not Funny theorized that the FCC filing hints at a "refresh of their [Sonos's] entire speaker line," as well as a touch surface or button of some kind to activate the voice assistant. Otherwise, the report is heavily redacted, leaving details sparse. The snippet referencing the new Sonos speaker reads as follows:
According to this policy, the unannounced Sonos speaker will continuously monitor the ambient noise of a home for command terminology spoken by the user, "without retaining or transmitting any voice recordings." The device will notify the user that it is recording thanks to a "visual indicator such as a light on the Product."
If Sonos does enter the smart speaker market, it'll be at a busy time for new voice-controlled home speakers. Amazon is rumored to be working on an Echo successor that would more directly compete with Apple's HomePod. Because Apple billed the HomePod as a high-quality music playback device first and foremost, sources close to Amazon's product development have mentioned that the company is focusing on significantly improving the Echo's sound quality, as well as enhancing its far-field voice technology.
Update 8/29: Sonos has sent out media invitations for an event on October 4. The invite includes artwork related to a mouth, hinting that the company's voice-controlled smart speaker could debut at that time.