The Linguistic Data Consortium at UPenn https://www.ldc.upenn.edu/ has a huge inventory of speech databases. I think the Articulation Index (catalog number LCD2005S22 and ISBN 1-58563-346-1) provides material that would meet both of your aims. It has 20 speakers with a set of approximately 2000 syllables in common to all speakers and approximately 20 unique syllables spoken by only a single speaker. The syllables were spoken both in the middle of a carrier phrase and in “isolation” (i.e., after a short pause at the end of the carrier phrase). The recordings were manually segmented to isolate the target syllable. There is also approximately 10 minute of conversational speech for each talker.
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