The most common cause of this issue is due to the wrong selection of audio output port. The WiiM Mini only outputs the audio in one of its analog audio (Aux-out) or digital optical port. When the Aux-out port is plugged with the audio cable, it'll be automatically switched to Aux-out.
Check the active audio output
You can check the active output port by selecting the Device tab > Device setting gear icon > Audio output and make sure you're using the correct audio output and connect the audio cable tightly.
Check the optical signal and audio resolution with test audio
If you choose the active audio output as the SPDIF optical output, please check if can see 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 contact us or your seller.
If there's red laser light but you still can't hear sound, please try to play the test sound on the WiiM Home app. Please select Device tab > tap Device setting gear icon > Audio Settings > SPDIF output resolution. If you can't hear 192 kHz, 24-bit audio, please choose 96kHz and see if you can hear the test audio.
Select the sample rate and bit depth pair and see what's the maximal audio resolution your system can support.