Both Cfgradio and Cfgsound will work using a stringtable.
The best way to make sure you've got it right is to de'PBO someone elses mission and have a look at their code.
You can also put your text straight into the .ext file without needing a stringtable... personally, I prefer a stringtable as it keeps all my text in one place, but it's up to you
You can also have a 'blank' in the title, which means the sound (or radio call) will play without anything showing on the screen. Then you can use radiochat (or sidechat) if you want to display other text (using format command or something).
I often use that if I want to insert the player's name, but have a blank in the sound file