Why it plays 30s sounds

“Alarms” doesn’t stay connected to Spotify overnight and, because of that, it can’t play a full sound at alarm time. Instead, it plays the short version of the sound you choose (saved when you pick it) on loop. Spotify chooses the short version, that's why you can't choose a specific part of the song to play.

All of this makes it possible to schedule alarms and use features like multiple alarms.

If you want the full version of your songs and playlists with gradual volume, use Smart Wake, that can do that by keeping the Spotify connection alive overnight.