Aside from other "hints," ask another station to monitor the repeater INPUT frequency, to see if you are having audio modulation problems, or just insufficient signal strength into the repeater. If you truly have an audio problem, standing right under the repeater won't help. P.S. Listening on the repeater OUTPUT really doesn't help, because that IS the repeater output, not your signal as it is received by the repeater. P.P.S. Some hams report ("gently, carefully!") enlarging the hole for the mic a bit increases the audio pick up; you do so at your own risk, however.