Microsoft BASIC has a 72-character input buffer. When that buffer overflows, BASIC tries to output a beep (BELL, ASCII 7). That character is the right half of a leftward-going submarine, character 7.
Almost certainly, this is not a problem with your computer. If the FLAC file is synthetic, it's possible that it could be missing some characters. Try generating the files yourself.
David Beazley has a python program to encode KCS here:
http://www.dabeaz.com/py-kcs/index.html
If you like FORTH, ED has a FORTH program to do it. Sadly, his page appears to have gone dark, but it is mirrored here:
http://dxforth.mirrors.minimaltype.com
Microsoft BASIC does a crude memory test on cold start, similar to what you describe. If it reports 7423 bytes free on startup, you have 8K of RAM present and probably good (but not thoroughly tested.)
Cheers,
Dave
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