Over the weekend, Microsoft published a support article describing an issue with a recent Windows 10 update that causes Bluetooth speakers to not work. Specifically, the article says that the issue is present in KB4505903, or Windows 10 build 18362.267, an update for Windows 10 version 1903.
The issue should only occur on PCs that have an internal speaker and one of three things will happen. Either your Bluetooth speaker won't connect, it will connect and the audio is of poor quality, or it will connect and audio will still play from the internal speaker.
If any of these are an issue, you'll be able to recognize it in the Device Manager. Under 'Sound, video, and game controllers', you'll find Microsoft Bluetooth A2dp Source has a warning icon next to it.
Luckily, there's a workaround. First, you'll need to open a command window with administrative privileges. Then you can run 'sfc /scannow', and when it's done, reboot your PC. This should repair missing or corrupted files.
KB4505903 was originally released on July 26 as an optional update. If you chose not to take it, everything included should have been bundled into KB4512508, this month's mandatory Patch Tuesday update. It's unclear if the issue is still present in build 18362.295, since it's not listed as a fix or known issue on the official changelog.