When installing a new Blackmagic driver for the Blackmagic Declink Video IO board, the driver also sets by default the PC sound card on the Blackmagic.
MIST and ICE outputs audio in parallel on the SDI and the PC sound card, and if both are set on the Declink, this will create an echo.
To prevent that, after installing the Blackmagic driver, go in the Windows setting and select another device for the default sound card, or deactivate the audio output on the PC sound card for MIST and ICE.