I have to agree with using the Gym Boss. I just set it and it repeats for the desired amount of rounds and I like the ability to be able to shut it off to transition between the rounds as I see fit.
I also use Tabata soundtracks for my kickboxing classes. Many of the online fitness music sites now offer interval music.
Found another app!
Sometimes you may require more than 2 intervals per round. For that You can use a free app (I have an android phone) called ‘A HIIT Interval Timer’. It allows you to set up mutliple intervals per round and also save certain interval timings so you can call them up again easily next time you use it. Highly recommended to those with android.