You are right, the problem actually only occurs when we have a combination of 480px2, 858 and TX Mode HDMI (problem disappears when using DVI obviously), not simply when using 858. Here is my report posted back in March on GitHub issue #34 :
I would like to report I am experiencing the same audio cuts problem reported here running fw 0.85a.
More specifically when using 480p x2 with H.Samplerate value of 858.00, let me explain :
If I switch to 480p passthru (still maintaining H.Samplerate value of 858.00) audio works perfectly fine.
If I stay in 480p x2 mode and change the H.Samplerate value by a few units, audio works perfectly fine but then obviously I get a blurry signal.
If I stay in 480p x2 mode and enable Allowupsample2x then change the H. Samplerate value to 857.95 or 858.25 audio works fine.
Tested on both AV1 and AV3 inputs.
I like your idea of the using the test pattern but it seems it is not possible to set it to 480px2.
Regarding my previous statement of not being able to get 1440px width I found out that it was because of my particular Adv. timing settings. When I used the test image you provided that had default Adv. timing settings 480px2 was effectively identified as 1440px by the TV, apologies for the confusion caused by this.
Let me know if you have any other suggestions I could try to hopefully identify the root cause of this issue. For the time being I will happily continue playing the audio out of the OSSC through some external speakers rather than through HDMI.
Once again kiitos for taking the time to look into this particular issue, it is a true pleasure to be able to directly converse with you on this hardware gem you created.