Adding 'audio' session type for Plex based on 'track' Plex API response type.

src/tools/server/sdk/plex/plexClient.ts

@@ -104,6 +104,8 @@ export class PlexClient {
         return 'movie';
       case 'episode':
         return 'video';
+      case 'track':
+        return 'audio';
       default:
         return undefined;
     }