Ingame voice acting

Wrong part of the forums, and not illegal, but if things go wrong, Riot isn't liable.

Not inherently, but you have to be wary of the risks if you're using a 3rd party application to do it; it must not provide other features which may be off limits (you'll be banned even if you don't use that feature), and Riot can't be held liable if it interferes with your client or gives you a virus or causes your account to be compromised.

You can refer to these posts for guidelines on what modifications Riot is generally ok with, and Riot's official 3rd party app stance;

http://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/5dmkJinG-3rd-party-applicationsmodifications-whats-allowed-and-what-isnt

http://boards.na.leagueoflegends.com/en/c/miscellaneous/5kx4F7VE-reminder-third-party-applications