Hi Ian,
It's not until you climb inside your headphones that you realise just how loud the saxophone is
Okay onto your problem:
In GarageBand, the individual track volumes on the left had side of the timeline enable you to create your desired monitoring mix by individually adjust the volume of the backing track and the track your recording into. You can readjust these after your recording when mixing down. So you might want to pull down the volume on the track your recording into to see if that helps.
However, I suspect that this might only get you part way there, as it could be the headphone amp in your mac or speakers just isn't powerful enough to bring the level up to match the acoustic level of your sax. There are several relatively cheap options that would give you a better headphone amp, I have one of these:
Behringer Headphone Amp but even a basic little audio mixer or old HiFi amp with a headphone out socket would do the job.
The other thing to consider are the headphones, if we are talking about the big over your ear jobs then they come in either 'open' or 'closed' versions, open headphones in their very nature allow external sounds in, closed tend to block out external sounds and are the better choice for recording.
Let me know how you get on,
Chris
I've just looked on the web and that headphone amp is much more now than when I bought it, for a little less there is this little mixer:
XENYX 502 but I'm sure there must be cheaper alternatives.