I was looking on here to get my dog to howl and saw that people tried to play clips online of whistling.
I found this site: http://soundbible.com/tags-whistle.html and used the DOG WHISTLE one, just to see if I could here it and my dog on my open nintendo 3ds on my bed started howling INSTANTLY.:) It should work for you too!
Deleted my post hours later as I came back and realised it could offend - certainly was totally unintentional but text you know, is a very funny medium to communicate in, so many pitfalls. What I said made me laugh, but might not others.
Suffice to say I tried the site again and could get nothing but silence out of the dog whistle sound, so I'm puzzled. I wouldn't have thought a real dog whistle would work, a toy whistle might, but I have no problem with whistling so hopefully when I come to it (If I ever do) I'll make my dog howl.
It's official; Eve is just stubborn - or maybe tone deaf. I got Rhett's Howl option to come up, called him over, nerved myself to try endless times to no avail, and whistled at him - immediately he threw back his head and howled. Speak command. Okay, but I did get Eve to do it, once. I whistled again - Rhett howled. (Speak command.) I whistled a third time - Rhett howled. Speak command - name trick - trick learned. So my second dog now knows all the commands, while my first puppy won't howl for love nor treats. And certainly not for whistles. Sigh.