Why did Paul want to be present with the Galatians and change his voice?
Paul wanted to be present with the Galatians and change his voice so they would truly hear what he was saying. He stood in doubt of them because they were listening to false teachers and falling for that error, despite his deep concern that Christ be formed in them.
Les explains that Paul had spiritually birthed the Galatian believers through his Gospel, travailing again until Christ be formed in them, much like a mother's birth pangs. He was thrilled when they were transformed, but now they were in trouble because they were listening to false teachers. So Paul desired to be present with them and change his voice so they would hear what he was saying, since he stood in doubt of them because they were falling for this stuff. He then challenges those who desired to be under the Law and to do something in addition to the Gospel.
I desire to be present with you now, and to change my voice; for I stand in doubt of you.