Plug in Microphone and sign in on controller... no Dj board needed.
Rock Band, Karaoke Revolution, and Lips have seemed to master the vocal pickup system... but DJ Hero2 and Guitar Hero seem to have the toughest games to keep streaks on. Regardless, I got this in less than 10 minutes so it's not really impossible. Remember, you must treat this achievement more like a vocal achievement than a DJ achievement.
There are two genre's of vocals in most vocal games: Spoken vocals (rap) and Tonal vocals (melodic/musical). Melodic vocals look like long slender tubes that are placed in different length's and height levels to show how long notes should be sustained and how high and low the notes should be sung. Spoken vocals look like green dots. Spoken vocals are easiest to get high streaks on because all they require is sound to be registered. When a spoken vocal is missed it turns dark (black/grey) and when a vocal is hit it turns white. You want to see a lot of white! One word turns black and you will lose your streak. You can also look at the total right under your score and see how your streak is. Miss a note and it returns to zero.
On games requiring high vocal streaks, choose rap songs that have relatively few melodic vocals. In Quickplay, I chose the first option: Message & Jungle Boogie because the majority of the song is a long streak of rap.
The game does not register if you are saying exactly the right words, so if you cannot read the script fast enough... don't panic. CREATE A Repititive syllable to replace the words. "I used Dig-a-Dig-a.".. saying it at a rather fast tempo (sixteenth notes if you know musical subdivision) Don't worry about the first verse where there are some actual musical notes. But once the rap starts... get a good breath and keep the syllables coming.... Breathe at the ends of phrases... BUT MAKE THEM QUICK...
If you have a fast and strong enough tongue, keep the syllables going even when the rappers aren't rapping because sometimes they tend to jump in and start before I thought the words started. This will end your streak prematurely.
Melodic vocals are much harder to keep streaks going because the pitch element comes into play. Again, I HIGHLY RECOMMEND A RAP SONG FOR THIS ACHIEVEMENT! Be loud enough into the mike that sound can pick up the syllables.
Some people use fans blowing into mike to register sound on non-tonal vocals. I cannot confirm if this particular technique will work but if my suggestions did not work, then you might try this as a backup plan.