Hi guycarmeli, you may need to connect power adapter to a grounded outlet.
I hope this helps.
Thanks for your response Joe.
The outlet I'm using is grounded.
As I noted, when I connected for the first time everything sounded well, and only a day later the problem started, even though nothing has changed in my setup.
The sound going through HDMI works fine, so it seems to be just an issue with the analog port.