The claim that two different phonemic symbols can represent the identical acoustic sound is a matter of basic linguistic and phonetic definition (e.g., allophones or sounds shared across different writing systems/languages can be transcribed differently while having identical acoustic properties), so no citation is needed.
The claim refers to a definitional aspect of phonetics and phonology. Different transcription systems, broad vs. narrow transcription, or allophonic variations can result in different symbols representing the same acoustic realization. This is common knowledge in linguistics and requires no empirical confirmation from the provided literature.