To modify the deflect success sound, just replace flame_thrower_airblast_rocket_redirect.wav with your desired sound, maintaining the original naming.
To modify the alarm sound, just replace sentry_spot.wav with your desired sound, maintaining the original naming.

If you are making them yourself, ensure that your sounds are the correct format for Team Fortress 2:
-WAV file format, 16-bit
-Mono Audio (merged)
-Sample rate of 11025, 22050, or 44100 Hz 

If you do not know what the above means, refrain from making custom sounds as TF2 will
100% destroy your ears as a consequence of not being able to correctly read those files.