It means that your Instrument Library isn't installed correctly - what's playing in the preview window is only an audio file and not the actual instrument.
Do you have anything in HD/Library/Application Support/GarageBand/Instrument Library? If yes, try repairing permissions. If not, you'll have to re-install GarageBand.
I found a response on MacRumors forums, but it doesn't solve my issue:
"i had to move my instruments and the sampler files to my USER>Library>app support>G Band location.
It's either gonna be in your HD>Library>app support......or your USER>Library>App Support.....
the instruments are all in two folder, one for the exs files, and one for the actual .aiff files.
Just look around for where they are, and move em might work."
The Instrument Library is there, SOME of the instruments work properly. Some do not. All of those that don't just sound like a xylophone. Repairing permissions did not help, nor did reinstalling GarageBand.
It's as if GarageBand doesn't know what instrument to associate with what loop. Even if "Acoustic Guitar" is selected as the instrument, xylophones still play instead after the loop has been dragged to the main window.
I've searched endlessly and cannot find a solution. Multiple people are having this problem.
I am going to assume that this is a bug in the software and that this issue is currently unresolved.
Again, I see people with similar issues, but no resolutions available.
The software instruments are installed correctly. I'm wondering if I need to delete Garageband completely from the system and reinstall it from the App Store. This is the Garageband that came with the Mac mini when I purchased it.
I had the same problem and found a horrible fix!
I think the fault was caused by loading the apps from my old Leopard time machine back up onto my new Mac Book Air running Lion. This seems to have corrupted all my apple loops. At first I reinstalled Garage Band which didn't work. I then upgraded to the latest Garage Band which also didn't work. I then reinstalled Lion from the app store (which is free if you have already downloaded it) which again didn't work. My Mac came with Snow Leopard and so I took the next step, wiped the hard drive, started with a fresh install of Leopard, upgraded to Lion (no repeat charge) re-installed my purchase of the latest Garage Band and it finally worked!
Nightmare so I feel for anyone who finds this thread with the same problem!
Interesting to read this. Not the ideal solution I was hoping for, but a solution nonetheless.
In my case, this new Mac mini came with Mac OS X Lion, so essentially what needs to be done is Garageband, and all associated files, plists, et cetera, need to be deleted. File association is just problematic from the time machine restore, which is exactly what I did from my previous Mac mini.
At least it gives me hope.
I just experienced this same issue. I had done a clean install of Lion and then upgraded to iLife 11 with the newere GarageBand version. To fix the issue here is what worked for me. I opened a new file in GarageBand with just the default instrument of Piano. Then I double clicked on the instrument and attempted to change it to Grand Piano Live. A message displayed that I needed to run Software Update in order to install additional software instruments. So I ran Software Update and sure enough there was an item for GarageBand instruments. I downloaded and installed them then relaunched GarageBand and all my original sounds were there with the exception of those from JamPacks. I had to go back and reinstall those.
I don't think this is a bug but it is somewhat of a pain and not so easy to figure out.