No. Those three traces you are cutting are all originally connected to ground (since they are for the I²S digital audio data and clock input of the HDMI transmitter IC, which isn’t used without the upgrade board -> so the input pins are grounded… to avoid digital noise, i guess). While cutting the traces and installing the upgrade board (by soldering it to the digital audio input pins of the HDMI transmitter AND to the analog audio input pins of the SCART connector ← i hope you did the last part too), you are connecting the HDMI transmitter to the analog-to-digital converter IC of the upgrade board, which takes the analog audio from the SCART input, converts it to digital data and feeds the data into the HDMI transmitter IC.
However the analog audio pins of the SCART input are directly(!) connected to the 3.5mm audio output jack of your OSSC 1.5 too. And those directly connected traces aren’t cut or modified while installing the audio upgrade, since it’s simply not necessary.
Or to put everything in a simple diagram:
Long story short:
Having the analog audio output working after installing the audio upgrade board sadly proves nothing. But it may be helpful, if you post a picture of your installed upgrade board. It could be you simply forgot to connect something or connected something wrong…