The most common cause of this issue is the wrong audio output selection.
WiiM Mini can output the audio in one of its analog audio (Aux-out) or digital audio interface (Optical SPDIF). When the Aux-out interface is plugged into the audio cable, it'll be automatically switched to Aux-out.
WiiM Pro can output the audio in one of its three output interfaces: Line out, Optical SPDIF out, and Coaxial. Please check which audio output is actively used when there's no sound.
Check the active audio output
You can check the active audio output by selecting the Device tab > Device setting gear icon > Audio Output. Ensure you're using the correct audio output and connect the audio cable tightly.
You can also check the used audio output by going "Now Playing" UI, going to the bottom, and checking the active audio port you're using, as shown in the picture below.
Check the optical signal and output resolution using optical SPDIF or Coaxial output.
If you choose the SPDIF optical output, please check if you can see the red laser light from the optical port (please don't watch it directly) if you're playing music. If there's no red laser light, the port may be broken, and please get in touch with us or your seller.
If there's red laser light, but you still can't hear the sound, please try to play the test sound on the WiiM Home app. Please select the device tab > tap device setting gear icon > Audio Settings > SPDIF Output resolution.
If you can't hear 192kHz and 24-bit audio, please choose 96 kHz and see if you can hear the test audio. You can select the sample rate and bit depth pair and see the maximum audio resolution your system can support.