The song that plays during the “Burn the drug fields” mission

What you've linked to is actually the Mjolnir Mix featured in Halo 2. It is a remix of the original Halo theme, from Halo 1.

In Halo 3, there are a few songs you could potentially consider "theme songs". They are Never Forget, Finish the Fight, and arguably Behold a Pale Horse, which contains samplings from many other songs in the series. A remix of the original theme is also used and it is One Final Effort. A slightly different version of this can be heard in the Warthog section of the last level.

The reason you aren't finding any boars is because there are very few areas where they are common. For example I've unlocked 8 Radio Towers and have only opened up one area that has them.

Here is the the best screenshot I could take that should allow you to figure out how to get to the boars. Also pointed out what a pig is so you know the difference.

^{Click on image below to view the larger, original size image.}

also here is a close up of a boar and pig