Hmm I keep meaning to add that list to the site but I always forget - thanks for reminding me.
Before scaling, all courses are initially standardised to a mean of 25 and a standard deviation of 12 (on a 1-unit basis). The scaled mean is the average course mark
after scaling has taken place.
For your example - the HSC mark that corresponds to the 50th percentile is 45.6 (not 48). This means that 50% of candidates received 48 or higher as their HSC mark. Last year, a 45.6 in Music Extension was scaled down to 33.8. This means that that same group of candidates received a scaled mark of 33.8 or higher.
33.8 is very close to the scaled mean, which was 34.3. Just under 50% of candidates would have ended up with a scaled mark higher than the mean.