Hi gdean,
If I understand correctly, you are trying to get Siri to play only music that you have stored on your iPhone. The resource below provides some information for working with Siri and the new Music app. Click on the link for complete details, but I've highlighted some key points here for you:
Play your music
When you play a song, you see the Miniplayer at the bottom of the screen. You can keep browsing other titles to find the perfect next song.
Or you can tap or swipe up on the Miniplayer to see the Now Playing screen and controls. Tap the 
in the upper-left corner or swipe down to close the Now Playing screen.
If you want to listen to an artist's entire collection of music that you've saved to your library, find the artist in My Music. To the right of the artist's name, tap 
, then tap Add to Up Next. If you want to listen to all of an artist's songs even if you don't have their music in your library, ask Siri to play all of their songs.
Use Siri
With iOS 8.4, Siri takes your music requests. You can ask Siri at any time to “play the top songs from 2015” or “play more songs like this.” You can even ask Siri to “add the new Blur album to my library."
Ask Siri to "play all songs by Pharell Williams" to listen to all of Pharell's music, even if it's not in your library, or "play my music by Drake" to listen to Drake's songs that you have saved to your library. You can also ask Siri to shuffle the songs in an album or playlist while you're listening to it.
Create a playlist
You can create playlists to listen to or share with friends. If you don't see a Playlists tab, tap My Music, then tap Playlists at the top of the screen.
Under All Playlists, tap New at the right of the screen, above the list of playlists. Fill in details for your playlist, such as the title and description, then add songs. If you like, you can also add an image. When you're happy with your playlist, tap Done in the upper-right corner.
Listen offline
You can save songs—including songs that you bought from the iTunes Store, added from Apple Music, or that were matched or uploaded with Apple Music or iTunes Match—to your device for offline listening. Tap next to the item. Then tap Make Available Offline to save them to your device.
To see music that you saved to listen to offline, go to My Music. Under the Recently Added section, tap the category selector (Artists, Albums, etc.). Show Music Available Offline is at the bottom of the category list. When this setting is on, My Music shows only songs that you saved to your device. Turn this setting off to see all of your music, including music stored in iCloud Music Library.
Hope this helps ...
