While connecting Plex to Soundiiz; you can at the first login step specify the "IP:PORT" configuration (or leave it empty) and a "Server Name" (can also be empty to select the default server), which could be very useful if your Plex login has access to multiple Plex servers.
Be sure to enter the correct server name in this "SERVERNAME" section before proceeding to the following steps in Plex.tv login step.
You can also specify sections or libraries you want to load with Soundiiz by providing a comma-separated list of section IDs in the "SECTIONIDS" field. You can, of course, load only one section or library, or leave this field empty to search music items in all your sections.