English only has voiceless stops after /s/ at the beginning of a syllable.
We say that the voiceless-voiced distinction is neutralized after /s/ -
there is no contrast between /st/ and /sd/, /sk/ and /sg/, /sp/ and
/sb/.
/st/, /sk/ and /sp/ might sound like /sd/, /sg/ or /sb/ sometimes.
After /s/, the voiceless stops /t/, /k/, and /p/ are unaspirated and
are phonetically very similar to the voiced stops /d/ /g/ and /b/. But
they are definitely the phonemes /t/, /k/ and /p/, however they are
actually pronounced.
As a native speaker, I'm not even sure how to say "exbect" and make it sound any different than "expect".
Hope that makes sense.