Zynthian: add minor troubleshoot case

zynthian/reterminal-install.md

@@ -122,7 +122,7 @@ Save the file (press `Ctrl + X`, then `Y`, then `Enter`), and restart the Zynthi
 systemctl restart zynthian
 ```
 
-Press the mute button to check if the touch screen is working. Or press in the upper left corner to open the main menu. If it works, **give yourself a pat on the back!** ;) 
+Press the mute button to check if the touch screen is working. Or press in the upper left corner to open the main menu. If it works, **give yourself a pat on the back!** ;)
 
 Next step? The sound system.
 
@@ -196,6 +196,11 @@ Now, you can reboot as instructed. Once the system is up again, go to the web in
 ![Alt text](key-mappings.png)
 
 
+# Troubleshooting
+
+* When using an external USB sound card, you may find that jack does not work properly: the command `jack_wait -c` does not return `running`, and the Zynthian UI shows an error message. Sometimes, this may be fixed disabling the sound card input port. Open the webconf tool, and go to `Hardware > Audio`. There, in the *Jackd Options* field, add `-P` after `-d alsa`. Reboot to test if it worked.
+
+
 # References
 
 ### Zynthian