At my friend Andrew’s wedding last year, I was terribly pleased with myself for discovering a great way to do cross-fades on track changes on iTunes using my iPhone and Remote.app.
The problem was that I had to switch between tracks on cue by cross-fading between them — usually not a problem if I just let the track end (there’s a crossfade option in iTunes), but not so easy if I had to fade out the current track before switching to the next one. The volume control on the Mac with its “clack” sound would have been loud and obvious (and was in discrete steps, so it wouldn’t be a continuous fade), and fumbling around with the volume slider in iTunes, selecting the next track and then re-upping the volume again would have been a bit tough.
Remote.app for iPhone or iPod touch (free, info here) solves this problem nicely, though, with its touch-sensitive volume control sliders.
To begin, make sure the computer with iTunes is on the same Wi-fi network as the iPhone (set up an ad hoc one on a Mac if necessary). Afterward, just set up a playlist with the songs in sequence and start playing; when it’s time to fade out, drag the volume slider down, hit Fast-Forward to get to the next song, then drag the volume slider back up. Easy enough. Hope that helps someone.