Fix beatmap preview tracks never using API sourced URL

osu.Game/Audio/PreviewTrackManager.cs

@@ -107,7 +107,13 @@ protected override void LoadComplete()
                     Logger.Log($"A {nameof(PreviewTrack)} was created without a containing {nameof(IPreviewTrackOwner)}. An owner should be added for correct behaviour.");
             }
 
-            protected override Track GetTrack() => trackManager.Get($"https://b.ppy.sh/preview/{beatmapSetInfo.OnlineID}.mp3");
+            protected override Track GetTrack()
+            {
+                string? url = (beatmapSetInfo as IBeatmapSetOnlineInfo)?.Preview;
+                if (string.IsNullOrEmpty(url))
+                    url = $"https://b.ppy.sh/preview/{beatmapSetInfo.OnlineID}.mp3";
+                return trackManager.Get(url);
+            }
         }
     }
 }

osu.Game/Beatmaps/IBeatmapSetOnlineInfo.cs

@@ -51,7 +51,7 @@ public interface IBeatmapSetOnlineInfo
         BeatmapSetOnlineCovers Covers { get; }
 
         /// <summary>
-        /// A small sample clip of this beatmap set's song.
+        /// A URL containing a small sample clip of this beatmap set's song.
         /// </summary>
         string Preview { get; }