For example speakjet.println("Test\375"); that works but if i type Test\375 from the serial monitor it speaks the \ 3 7 5.
When the compiler runs, it sees the \375 in the string, and substitutes a different value. The string that is passed to the speakjet.println function does not contain a \ and the characters 3, 7, and 5.
In your code to read from the serial port, you need to make that same substitution. The value following the \ is an octal value.
375 base 8 is 253 base 10.