Protecting SMS Messages with TextSecure
There is an open source application called TextSecure (source) that encrypts SMS messages. However both clients must have the app for it to work as the encryption is done on the device so there is no need for any intermediary servers you need to trust. You must first setup a secure session by exchanging encryption keys.
The effectiveness can be tested by setting up a secure session two phones, then removing or disabling TextSecure off one of the phones. SMS messages coming from the phone with TextSecure will be unreadable garbled text.
Early versions of TextSecure had a bug where unencrypted SMS messages were output to the phones logs, exposing information that otherwise would be kept safe in the SMS database. The latest version however fixes this issue.
Embed text messages in an audio file
StegDroid was developed as part of a university project by Tom Medley. It was featured on the XDA developers site and there is also a thread on their forums there.
It allows you to embed text in an ogg file and optionally password protect the text. Users with the StegDroid app can extract the text. It's a bit heavy for everyday use though.
Protecting voice calls
There is really only a way to do this with VoIP. Governments are wanting access to tap VoIP data and Microsoft gave them this with Skype. Fuck these guys, there is an application called RedPhone that encrypts the VoIP data so it cannot be MITM'd. It is also open source and made by the same people as TextSecure.
Again both phones need to have the app for the encryption to work.
There is also a commercial closed source app called Kryptos. It costs $10 a month and claims military grade encryption (AES256 encryption with RSA2048 key exchange). It also provides interoperabilty with iPhone users because there is an iPhone version of it. However being closed source makes it less trustworthy as we can't inspect it for NSA backdoors or exploits.
Of course there is a lot more you can do to secure your Android like rooting it and blocking ad servers but I'll leave that for another post.
I would advise anyone interesting in privacy and protecting themselves to checkout r/netsec, r/privacy and r/cyberlaws.