On CrossOver 17 and greater, the simplest solution to fix this issue is to run: /path/to/cxoffice/bin/cxfix missinggstreamer1mpg123audiodec missinggstreamer1mpg123audiodec.amd64
Or add --show-all to see how CrossOver would fix it on your platform and others.
Alternatively you may try one of the commands below.
For 32-bit Windows applications
32-bit Arch Linux : pacman -Syu gst-plugins-good
64-bit Arch Linux : pacman -Syu lib32-gst-plugins-good
The commands usually also apply to derived distributions. So for instance a fix for Debian is typically also applicable to Ubuntu, Linux Mint, Deepin and Pop! OS. Similarly a fix for Fedora likely also works on Red Hat Enterprise Linux, CentOS, etc.
(1) Before Debian 10 the package containing the 32-bit plugins is incompatible with the 64-bit one. This also impacts the derived distributions such as Ubuntu, Pop OS!, etc. On affected distributions you will have to choose between the 32-bit plugins (for 32-bit Windows applications) and the 64-bit plugins (for your Linux applications such as Firefox).
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