Owners can use it to listen to Spotify, iPlayer Radio and other selected apps on Android and iOS devices, and audio streamed via the Chrome browser on PCs.
The Chromecast Audio dongle costs £30, making it cheaper than many other similar streaming add-ons.
It also poses a challenge to speakers with built-in wi-fi, whose makers often charge a premium for their products.
At an event in San Francisco, Google said it had sold more than 20 million units of the original Chromecast, which streams media to TVs.
“The original Chromecast did really well because of Google’s brand, which meant people trusted it would do what it said on the tin,” said Stuart Miles, founder of the Pocket-lint tech website.
“We have seen similar devices to this before.
“But it will probably be quite successful because it will encourage people to upgrade their old hi-fis – which are still very good from an audio perspective.”
The launch coincides with the release of a flagship “smart speaker” from Sonos, one of the bestselling brands in the sector.
Its kit is many times more expensive than Google’s accessory, but Sonos says it delivers a superior “room-filling listening experience” that can be fine-tuned by new software.
Google also announced a second-generation Chromecast TV media streamer, which now has a third antenna to create more robust data connections and supports more apps than before.
The products are going on sale at a time when Apple, Amazon and Roku are also launching audio-visual streaming devices of their own.
Source: BBC News