If I understand the issue...
Mycroft uses the tool "mpg123" to play the mp3 with the NPR news, skill "npr_news" (see mycroft.conf, line "play_mp3_cmdline").
The answer to "what's the weather" uses another output channel. It is not based in "mpg123" nor in "mimic -t".
When mimic is configured as tts, it uses "aplay" command (see "play_wav_cmdline" in config) to play the result of the tts conversion.
I suggest you test with this command. Could be it is need add an argument "-D" to the command.