when you use SideRadio you're telling it to play a designated sound file.. and display the accompanying text on the screen if i'm not mistaken
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Command :
SideRadio
Description :
Allows a unit to say something over the radio. All units on the same side of the speaker will receive this radio.
Syntax :
unitName SideRadio "CfgRadioClass"
unitName is the name of the unit to do the talking.
CfgRadioClass is the name of a CfgRadio class that you define in your mission's description.ext file.
Returns :
N/A
Example :
Unit1 sideRadio "RFallback";
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So when you use that syntax you posted.. OFP is looking for a file named:
Radio message Roger that Alpha. The forest areas are heavily guarded. Stay sharp!
Do you want text to appear on the screen only? or do you want a sound file to be played as well?