Fla's - sid , ek - sept

Succinct is pronounced "suk SINGKT", with greater emphasis on the second syllable, much like the emphasis used when saying "ballet" and "conform". Flaccid rhymes with acid and placid. FLA sid. Accept - The first syllable sounds like 9 parts "ack"and one part "eck". Emphasis is on the second syllable: Ak SEPT

Suk-SINked FLAsid (like acid) ack-SEPT (like "back" and SEPTember)

'Accent' also has stress on the first syllable, and certainly is pronounced "AK-sent". The 'k' sound remains when the stress is changed in 'accentuate', which is pronounced "ak-SEN-tu-ate". The discrepancy with the double consonant (in English words of Latin origin, whether directly or circuitously) is that a 'c' is always pronounced like a 'k' unless it is followed by a soft vowel, ie: "SIR-kus" (circus). Thus, the first 'c' is the regular 'k' sound, the second 'c', followed by a soft vowel, is pronounced like an 's', "FLAK-sid". Notice that when 'cc' is followed by a hard vowel (a,o,u) or another consonant, it is usually pronounced as a 'k'... "AK-kur-ate" (accurate), "ak-KLAME" (acclaim), "ak-KORD" (accord). The story with a word like "SOK-ker" (soccer) deserves more discussion! Even without the double 'cc', English has tried to hang on to this rule - I hesitate, but introduce the pronunciation of the noun and adjective of the same word, "FEE-seez" and "FEE-kal" (feces, fecal). The answer is that while "FLAK-sid" was once, and still is the "ak-SEP-tid" pronunciation in some of the English speaking world, "FLAS-sid" seems to have "suk-SEED-ed" it in the United States.

In English, double "c"s can be difficult. I can't tell you any rhyme or reason for it, but in your examples, the first & last have a "hard c" or "k" sound followed by the "s" sound, whereas in "flaccid" both "c"s are pronounced like "s"s.