Voiceless consonant -> voiced consonant before vowel?
In a comment John Lawler wrote:
It's not that one thing becomes another. It's that the muscles that create the consonant and the muscles that create the voiced vowel are independently articulated, and therefore their relative timings are not always coordinated precisely. Voice Onset Time (VOT) varies greatly from individual to individual, and from sentence to sentence by the same individual. So sometimes it does and sometimes it doesn't is probly the best answer.