A few posts on this topic, this is a detailed one http://www.elevenplusexams.co.uk/forum/11plus/viewtopic.php?f=18&t=25621
The test is recorded and is known as the Bentley test. The chord test is done with std intervals but the test for pitch lower and higher is comprised of notes much less than a semitone. Some are pretty hard to determine.
I tested both my DC's initially with no practice and then after some practice a couple of months later. There was very little difference and my view is that you can either do it or you can't. To do something I would practice playing chords with 2,3 and 4 notes and get DS to answer, just do half a dozen a day, should only take a couple of minutes.
The answer paper is in 4 sections,
20 questions on pitch you just indicate the second note is above, below or the same the first note
10 on tune recognition where you indicate the note that was different in the second playing of the tune
20 on chords where you identify how many are notes in the chord
10 on rhythm, you identify if the two rhythm patterns are the same or if different, which beat is different.
The test is well explained at the outset and I have not heard anyone being confused by the test.
Sorry forgot to say, the test is played through a sound system not headphones.