I have posted a simple protocol server for Windows on GitHub: https://github.com/kaytat/SimpleProtocolServer
There is more technical info in the README of that project.
The server will probably require a little configuration from the user. The reason is the WSAPI loopback used for capture does not allow for programmatically changing the sampling rate. Since the SPP player requires a certain format, the user will have to set the properties of the playback device to match.
Here are the steps to do so for a Win7 machine. First open up the “Playback devices” from the system tray.
Choose the “Properties” button of the sound card being used:
Choose the format the best fits SPP. SPP defaults to 16bit 44.1kHz, but it will also support 16bit 48kHz.
A corresponding update to SPP has been posted on the Play Store that adds an option to choose either 44.1kHz or 48kHz for the sampling rate.