Sometimes when you do a RAM preview the audio will not sync with the video, especially if you have effects applied to your layers. You could render a portion of the video and view it in realtime. However if you are playing back your ram preview and your info palet states that it is playing realtime, you may need to tweak the placement of the clip. Try selecting your clip (with the audio) and press LL. This should bring up your audio waveform for that layer. Use it to “see” your audio and slide the clip until your audio syncs.
Good Luck