I'm a sound engineer for the play my school is doing. At one point, which the director has warned me could vary each night, the music that will be playing out of my laptop and into the mixing desk will need to slow down, as the characters take some drugs and it needs to get a bit psychedelic and weird. I don't know, however, how best to manipulate this speed change? The only idea I've have is to use my budget to buy a digital DJ deck/turntable which has a speed control on it and run the music through that. If that's the best option, can anyone recommend a turntable?
Since you're working with an audio file, not "live" sound...and you appear to be trying to do this with no budget...you can download Audacity (free open source program) and do time manipulation to the file within it. Work with the director to get the desired effect, then output the file. You'll have something predictable and repeatable for the performers that way.
This video should give you most of the info you need to pull it off:
If it were me, I would prepare a separate audio file for the slow motion cue, and make sure that the regular speed cue before it is considerably longer than it needs to be. And then trigger the crossfade when the action onstage goes slow motion.
In my experience, that kind of loose instruction from a director often just means, "I don't know exactly when yet. This is hard for a sound designer to plan around! But if you are manipulating the slowdown on the fly, it will (likely) be completely different every performance. In some cases, this is the desired effect, and the director and actors like the spontaneity. Again, in my experience, actors like some predictability from the music. Music can then be a familiar guide to lead them through the story.
Just my thought.
If the music is going hyper slow, like Paul Stretch slow, then it won't matter too much if it triggers at EXACTLY the same part of the music pre-slow down.
You could just run the entire mix from a DAW and perform the pitch shift there using a real-time pitch shifter plug-in (there are many ways you could do this in terms of mixing and control). This is probably the most easiest way, given that computers are so commonly used for playback anyways (in addition to decks, which you don't seem to be using).
You may need to try out how it sounds.
You can use the 'Rate' fader in Reaper for on-the-fly rate changes. Using a MIDI or OSC fader may help to smooth it out and reduce pops (not sure), but in any case you can type in a new value and press Enter on the cue.
The Rate parameter can be found just next to the transport. You can try Reaper for free, if you don't have it.