I have spliced poses together within Poser and can not see what you are doing wrong. However my method varies slightly. At the end of the first animation I advance one frame, select the pose and click once on the check mark. I don't double click. Also I have not tried it with sound. If you still end up with your 30 frames, try doing one without sound.
Not, I appreciate a conclusive answer, but hope it helps a little.