Hi Kathy,
This is definitely possible! There are two ways you should be able to do this.
 
First, you could simply open the Alexa app on your phone, and say “Alexa, set a sleep timer for 30 minutes.”
However, if you would like to expand or customize this command, you could use a Routine.

To access your routines, open your Amazon Alexa app and select “More” at the bottom right corner of the screen, then tap on Routines.

1. Open Routines on the Alexa app, and press the “+” (plus button) in the top right corner to create a New Routine.
2. On the New Routine screen, tap the “+” (plus button) to Enter a Routine Name. Call it “Bedtime”, then push Next.
3. On the next screen, tap the “+” next to “When This Happens”, select “Voice”, and enter what verbal command when you want this routine to run. Enter “Bedtime”, then push Next. (Alternatively, instead of “Voice”, you could select “Schedule” and set it to happen at a specific time.)
4. On the next screen, tap the “+” next to “Add Action”, which opens up the “Add New” screen.
5. Scroll down to “Music”, and tap on it.
6. Under Play, enter what kind of music you want – Song, Artist, Playlist or Station. You can enter “Ocean Sounds” here.
7. Under “From”, select the Music Provider you normally use.
8. Under “Duration”, check the “Timer” box, and make sure it’s set to 30 minutes. Then push “Next”
9. Lastly, choose which device you want this routine to play music from – I would recommend picking “The device you speak to”.
10. Then push “save”. Your routine will take a minute before it starts working.

That should get you on your way to having a sleep timer that works the way you want it to!
-Daniel