remove the ) between the file name and the , separating the %j parameter.
Btw: looking at your code, a better way to do this is to use mIRC's /play command, as the way you're doing it will probably flood you off of the network, as the /play command has a built in delay
Also you don't need the / in the alias name.
alias pay_high {
.msg #BIGZIPZ Winners:
.play "C:\Program Files\mIRC_ComicQuiz\HiLo_HighBet.txt"
}
If you really want the word Winner in front of each line, then I suggest you change the msg line in your code to read
.timer 1 %j msg #BIGZIPZ Winner: $read("C:\Program Files\mIRC_ComicQuiz\HiLo_HighBet.txt",%j)