You would do this using movie studio. Bring the wav file into movie studio and add whatever additional audio you need to the audio track - then render it out to a wav file.

Another option would be to time stretch the audio - place the cursor over the right side of the clip (the cursor should change), hold down to CTRL key and stretch away.

Keep in mind that if you are also using video on the start page, that the audio and video should both be the same length.

--Scott