I recently acquired a HermesLite2 "Plus" variant which is an add-on board for HL2 that adds mic in and audio out capability to HL2. It also has CW input and sidetone via the phone jack. Pretty interesting since it reduces the PC dependency on audio a bit.The Plus variant is not supported by some of the SDR software. However, popular ones like PowerSDR, Thetis, pihpsdr etc support it.

After hooking it up with pihpsdr on GNU/Linux, I could hear my CW sidetone just fine. I could change CW parameters and they take effect. HL2Plus uses the "dither" on/off bit to signal the radio/FPGA that the audio needs to be sent to it. However it did not have any effect in my case. Audio would still come via PC headphones.

After asking the mailing list, Taka JI1UDD, one of the designers of the project replied with some modifications to be done (since HL2 does not have local audio playback facility, the software makes those buffers zero and this needs to be removed/commented out). After doing that, it worked just fine. Sidetone is nice and latency is very low.