Well I donâ€™t know about logic 7 but I can tell you how I do it in Logic 5.5
First load your sample up in to an audio track. Then you will want to open up the logic sound gate on to the audio tracks FX chain. On the sound-gate you will see a box at the bottom that is called side chain. Click on this box and a list will open. Select one of the busses numbered off this list. Next open up The ES1 or any Logic synth really but this one will work well so long as you have the volume of the ES1 turned up and the ES1â€™s envelope has short attack and release settings with sustain all the way up. Write a simple pattern for the ES1 this is going to be your trigger for the sound gate. Now that you have done this we will route it all up to trigger the sound gate.
Now on the audio instrument track that you have opened the ES1 on, change the output from output 1&2 to the bus number that you had selected for the sound-gates side chain. You do this by clicking on the output box, which you should see on the track. Now go to that bus in the environment and set is output to none in the same manner. You will no longer hear the ES1â€™s sound but it is now triggering the sound-gate. Now when you hit play you should hear the sample coming through only where you have written notes with the ES1. Hope I have explained this clearly enough for you. I didnâ€™t wake that up long ago so it might be a bit hazy.
Set up a noise gate on the vocal channel (in plugins, dynamic).
Then either set up some audio u want to trigger the gate on an audio line, once youve done this pull the volume level of the audio track right down.
Or you can use a synth (ES1 is good), change the output of the synth from output 1-2 to a bus then on the noise gate put that bus as the side chain. Pull the level of the bus right down.
Now fiddle with the settings on the noise gate untill you get the desired result.
Also remember if you solo the vocal track with the gate on it then you must solo the track that is triggering the gate otherwise you wont hear anything.