You're using GOTO with RETURN in the sub-routine. Try this version;
Code:
;************************************************* *******
CMCON = %00000111 'Port A all digital
CVRCON = %00000000 'CVref turned off
OPTION_REG = %10000111

Main:

low HoldTxON ; Initialize by clearing all the outputs
low LED
Low MotorControl_1
LOW MotorControl_2

pause 5000
gosub Lowvoltagecheck
goto Main

end
;************************************************* ******************************
; Subroutine to check for battery voltage at the beginning and end of
; transmission flashing to signify low voltage or high voltage

LowVoltageCheck:
If lowVoltage = 1 then ; If the battery voltage is OK
high led ; turn on the LED for 5 seconds
pause 5000
LOw LEd
else
for loop = 1 to 10 ; If the battery voltage is low
high led ; Flash the LED 10 times
Pause 500 ; at a rate discernable from the 
low led ; transmit LED flash rate
next
endif
return
;************************************************* *******