You need speakers. Although, I guess you could put the line-out of the DDJ-SX into the line-in on your laptop and monitor it.

If you're spending this kind of cash on a controller, spend some money on some powered monitors to get the most out of it.

Hi djsukh,

Yes you will need external speakers to plug into the rear of the DDJ-SX. You will not be able to use your computer speakers for Serato DJ as once the DDJ-SX is connected, it becomes the primary soundcard.

You are able to run cables from the rear of the DDJ-SX into the Line In input as akakak mentioned above, however I would not really recommend this. A powered speaker option will be a far better choice.

If you dont want to buy powered speakers, you can just run RCA cables out of the DDJ-SX and into any home theater or stereo system with auxiliary RCA inputs.

Cheers

For practicing or cue-ing ect, I just use my headphones to monitor. You're obviously not going into a party off of your laptop speakers, so (i'm using the WeGO but it's essentially the same for the SX).
The controller is plugged in by USB to the laptop, the controller's internal sound card is what Serato will use. as mentioned before for practicing and pre-work, I plug in my HDJ-1500s into the 1/4" headphone out port and set the cue/master to 100% master for just playing with the files, and at 50% cue 50% master for practicing. When showing off to my friends, I plug in an RCA to 1/8" AUX into the master out on my controller and plug that into my little iPhone speaker dock thing.
For actual gigs, I have a guy I work with who does speakers, that's what he does.
I plug my master RCA out into his master line in, and it then goes to an array of speakers.
Hope that was helpful and not to ramble-y
Cheers.