You can split it into separate sections and change each section independantly. You can also use Effects / Pitch / Bend in Sound Forge for smooth changes.
If you just want to play it by ear, you can hover your mouse arrow at the end of the audio section you want to slow down (split it on both sides of the section) and when you hold down the "Ctrl" key at the same time, you'll get a wavy line under your mouse marker. While holding the Ctrl key, you can drag the end of the audio clip to the right to slow it down, or to the left to speed it up. The pitch will remain the same, just slower or faster, I think up to 12 times. If that's not enough, you can render a new wave from that and do it again. (works with video too.)