mint-arena: Allow target_speaker to have have any sound extension

If no extension, default to .wav. Otherwise leave it be, it could be .ogg or .opus.
mint-arena: Make target_speaker without sound a soft error

Single player maps for SoF2, JK2, and JA often use some other key for sound.
Allow them to load anyway (assuming engine's been hacked up to load RBSP maps...).