A new Chromecast Audio with Bluetooth may have just hit the FCC, though it's worth noting that unfortunately, there isn't much information to dig up from the included documents. Short of the location of the FCC label, and the obvious visibility that the device is a Google device thanks to the letterhead, there is really no concrete information about what this device may be. The documents merely list that this is a Bluetooth device, but considering that companies tend to list products that pass through the FCC as a mobile device if they happen to be a phone, chances are this is not a new smartphone from Google.
That being said it is something that has the ability to connect to other devices through a Bluetooth connection, so perhaps it's a new speaker. A little further digging does reveal some other details, though they aren't very telling either in terms of what the device is. The model number for example is listed as A1JT, and doing a search for that brings up additional FCC information which also lists this device as a Digital Transmission System. This could mean that the device is some sort of Bluetooth transmitter, though there hasn't been any information about Google building such a device, so it could potentially be a new Chromecast device that has Bluetooth technology built-in.
If it is a Bluetooth transmitter, these types of devices are usually used in vehicles for connecting smartphones for audio playback. Considering this, it could be a Chromecast audio device for the car that allows people to stream music via Chromecast technology to their car stereo. This is of course complete speculation based on the very minimal detail that's available in the FCC documentation, but it would be an interesting next step in the advancement of Google's Chromecast lineup. Perhaps the more likely scenario is that this would just be a new version of the Chromecast audio that could be used in the home, this time building on what was already available by adding Bluetooth technology as a possible wireless connection type. So far there hasn't been any mention of Google coming up with a new Chromecast Audio to replace the currently available model, but the regular Chromecast has already been replaced twice, technically, if you count the 2nd Gen. model and the Chromecast Ultra as separate replacements, though truthfully the Chromecast Ultra is just a higher-tier version of the Chromecast 2nd Gen. designed to allow those who have 4K TVs to enjoy 4K streaming content. That being said, the original Chromecast Audio initially launched in September of 2015, so consumers are due for a new version and perhaps Google is preparing to unveil a new model this Fall. Having said all this, it is worth mentioning that there is next to no information what this device is, so it could really be anything, but a new Chromecast Audio seems like a logical possibility.