Ok let's start.
Unless you have some other pack installed ( like SAS modact or DBW and so on)
The selector options mean nothing. Up3 will be the one to load regardless of what you choose.
The selector can be usefull if you want to also have a 2nd modpack installed or want to change to stock.
(In other words selecting DBW in selector does not automatically launch DBW)
If you are wondering what each of these options for modded game offers,
1) SAS Modact and Classic modding mean that by using some tools(like the ones in SAS) you can build your own colection of mods.
But it's very hard to do ( not just a coincidence so few big packs exist) so better to stick to a big pack and put your mods on top of it.
2) UP3 of course you already know what it offers
3) DBW is specifically designed for offline use and is installed above UP. You might want to see this if you only play offline.
Now about the sound mods,
here is N1Ks thread: http://ultrapack.il2war.com/index.php/topic,5170.0.html
and here Jiver's one: http://www.sas1946.com/main/index.php/topic,16999.0.html
Tiger's UV2 might also be interesting for you as it works 100% with UP3 but ever since UV3 came out I can't find link (UV3 is very good but a lot of UP users had issues with it and not minor ones. I don't recommend it especially if new to modding scene.)