I was rendering to m2ts 1080-60i as my Panasonic HDC-SD10 shoots in 1080i. However, I found the problem in a way or another. I rendered the video in wmv format and everything turned to be well, the audio volume being the same as the song and preview one. There may be a problem with the mt2s format or AC3 codec. In conclusion, I used another format to render and everything it’s ok. Thank you for your support, Mike!