Final answer:
Sound numbering, accurately known as the Soundex system, is indeed a phonetic indexing system that allows surnames with similar pronunciations but different spellings to be filed together, confirming the statement as true.
Step-by-step explanation:
Sound numbering is a phonetic indexing system known as Soundex, which is designed to group surnames that sound alike but are spelled differently. The answer to the question 'Sound numbering is a phonetic indexing system allowing surnames that sound alike but are spelled differently to be filed together.' is true. The Soundex system codes surnames based on their pronunciation rather than their spelling, allowing for surnames with similar sounds but different spellings to be filed and retrieved collectively. This is particularly helpful in genealogy and government records where different spellings of the same name may be entered.