I finally got my UltraHDMI mod and can now reproduce the reported high frequency “loud broken screech” in the L+R menu. It happens roughly 50% of the time if you enter the L+R menu. It is only present over HDMI audio and is not present on the RCA Audio.
I debugged it down to the audio smoothness function of Neon64. Neon64 fills the audio buffer with the same byte, which creates silence over RCA, but a constant sound over the UltraHDMI Audio.
As a small workaround I am now clearing the audio buffer every time the L+R menu is entered, which creates a short “plop” sound, but no more constant sound.
The small fix is attached below as version 0.3c.
In short, if you are using RCA audio stay with 0.3b for audio smoothness and if you want to use HDMI audio try version 0.3c.
As a full fix, the libdragon audio pause silence interrupt handler could be ported to Neon64 or the UltraHDMI Audio could maybe behave similar to the RCA audio.