Is there a sound mod which gives the bow a proper twang when I shoot an arrow?

Are there any good landmass mods west of Vvardenfell apart from Silgrads tower and Wizards island?

Havish and Dulsya Island are two that I think of. Take a look at http://www.mwmythicmods.com/MAPoMODSbyCyronaut.jpg. I've never tried Velaya, but it looks interesting.

Cool I checked out Velayia on MMH, I get this message..

velayia.esm
Contains 94 dirty references - this mod will require cleaning before use.

What impact does dirty references have on gameplay? Will I get more error messages? Can I clean a mod with Wrye mash?

I used to use TESTool to clean the mods, I think that is still the way to go. Just don't use the other features it has. Perhaps there is a newer tool for cleaning. My Morrowind Mod Mix was cleaned years ago and any new mods I have gotten have not needed to be cleaned so I have not kept up to date on this.

As far cleaning dirty references, yes you want to clean those mods. (Note, things like Unofficial Patches and some certain mods should not be cleaned, but the readme's will be clear on that) While it is not an instant issue, these references can cumulatively add sludge (and corruption) to the game save over time if they don't just cause a crash when an item tries to reference something that is dirty.

See the following

http://www.uesp.net/wiki/Tes3Mod:Creating_Clean_Mods

https://moddinginmorrowind.wordpress.com/2012/07/23/modding-tutorials-cleaning-mods/

The only thing I use TESTool for is merged objects. You can clean mods with TES3cmd. If you have Yacobs's integrated Wrye Mash there is an option to clean with TES3cmd from within Wrye Mash.

A "dirty reference" means that the mod includes something that's the same as in one of its masters (an ESM file like Morrowind.esm). It could be a door in a an NPC's house for instance.

They may not be a problem but if a mod with a dirty reference (sorry, but I've just thought that that means double entendre doesn't it? Anyway...) loads after a mod that intentionally changes that reference then *bang* compatibility problems.

So, always best to clean your mods. TESTool is fine (ensure that Restricted Dialogue Cleaning is enabled) but I'd recommend tes3cmd.

As to sound changes to bows I can't think of a mod that does just that. You could try Intelligentsia's Marksman Mod, that adds a lot more content (good mod!). There's also Items Sounds Extension by Walther 2007 (not tried it)