You're in for a shock if you think he going to be in a position where he can counter Mayweather like he did Cotto, Mayweather's speed and ring IQ are on a completely different level. For Pacquiao to stand a chance of countering him on a regular basis he will have to be as patient as him and that will mean dropping his punch output right off, yet most see his work rate as his main weapon in beating Mayweather.
One of those random non-copyrighted ones that youtube provides on audioswap. BPM- Intro 81 Beats per Minute