Testing in Terminal produces the result:
curl -I -r 200-300 http://static.squarespace.com/static/51112ff9e4b04cd3eb15e570/t/51e04face4b0ab67 379c2a53/1373654956203/Pilot%20-%20%22Hear%20Comes%20Treble%20Guilty%20Plesures% 22.m4a
HTTP/1.1 200 OK
Cache-Control: public, max-age=94608000
Content-Type: audio/mp4a-latm
Date: Fri, 12 Jul 2013 22:13:17 GMT
ETag: W/"10c0f9ae9ce6b1142ec17ee5b9a6ad46"
Pragma: cache
Server: SSWS
Set-Cookie: JSESSIONID=AF83B0926CE292D14EF45426C48A03A8.nyc-webster11; Path=/; HttpOnly
Set-Cookie: crumb=2cfce6f116; Path=/
Set-Cookie: SS_MID=9a158195-38ca-4bc6-b220-0a6351bc4dafhj1xzdmp; Domain=.squarespace.com; Expires=Mon, 10-Jul-2023 22:13:18 GMT; Path=/
X-Identifier: static
X-PC-Date: Fri, 12 Jul 2013 22:13:18 GMT
X-PC-Host: nyc-webster11.int.peer1.squarespace.net
X-PC-Info: Qd1d-wv7whOp0c6_YVo-yjKegbg, [t:234, p:1718], 25972:16499, 0.01699525503671776
X-ServedBy: nyc-webster11.int.peer1.squarespace.net
Content-Length: 22262998
Terminal has requested only bytes 200 to 300 of the file, but the final lilne shows that the full length has been called.
This suggests that the server does not support byte range requests: this could well cause problems with iPhones (though if this is the case it's a mystery how you got it accepted in the first place as normally this issue causes a rejection with an error message about it).
Incidentally, though presumably this isn't the cause of the issue, your filename has spaces in it. Spaces are not allowed in a URL and though the feed correctly handles this by substituting the code %20 for each space, so that it does work, it's bad practice. You should replace the spaces with underscores and amend the feed accordingly.