Question: Are there any grammatically sound sentences in English where every word starts with the same letter? If there are many such sentences which is the longest one?

Answer: 

Absolutely. Assembling an appropriate answer appears achievable, assuming an articulate author appropriately adept at alliteration.

Behold, by being brave, but besides boldness by brainstorming before beginning, broadcasting brief blurbs becomes bizarre but basic babbling.

Continuing, casual crowd commenters can concur, collectively checking current compositional constraints controlling character choice, concerning certain crackpot creative chores, chiefly claiming common cunning's compelled conclusion, comprising conjectured chore conquerability (circumstantially) carrying clear caveats cautioning care, considering calling colossal commitments complete cinches could convey controversial cool confidence.

Don't dare doubt David's diligence doing deeds demanding deft, devious design, dear discussion denizen, deeming dogged determination doesn't darken David's door; during diction's driest drudgery, despite delightful daydreams delaying development, destiny's death-defying daredevil detail deviser dodges despair, denying defeat, displaying devotion demonstrating dreary deliberation doubles daft drama's devastating dynamic disposition, dovetailing directly.

Edit: People have asked for more, so here is another.

English enthusiasts eagerly envision enhanced emotional elevation, exceeding even existing examples' elicited elation, encountering each ensuing eccentric exercise, ergo everyone's esteemed enlisted essayist (enchanted!) ensures each exquisite excerpt exhibits explosive expressive efficacy evincing either excruciating editing effort, extreme endurance, engineering excellence, etc., else elemental extemporaneous effervescence, entertaining enough except -- explaining earnestly -- entailing eventual emergency; essentially, endorsing ever-escalating elaborate experimentation encourages extravagant excesses, especially emphasizing expectations encompassing elusive execution extending established events -- exclusively employing equal everyday emblems (E's, e.g.) embodying each emblem ensemble's earlier end (elsewhere enunciated easily) -- evaluating endowing entirely equivalent eloquence eternally, envisaging electronic education's emerging endeavor enjoying eight, eleven, even eighteen entries, exploits experts empirically estimate expending extra-Einstein egghead energy engendering environmentally evil, Earth-exposing exhaled exhaust emissions.

Five funny features feel fairly finished for forum fodder, foolish followers falsely fancy, for failing further focus fueling full foresight, fellow fiction fanciers frankly forget fundamental facts framing fruits from frenzied freelancing, for first fragments flowing from frantic freshman fingers frequently fall flat, forming forced, flawed fakeries feigning fluency, faded facsimiles fractionally fitting for fatuous Facebook flapdoodle, familiar formulaic fragments famously fouling friends' feeds; fine, for furthermore, fascinatingly, first forays facing far-flung fringe frontiers, finding faith's fortitude ferociously fighting formidable foes -- fear, frustration, flaky functioning, foot fungus -- forge foundations for future feats, figurative furniture for fortune's foyer, faintly favoring fantastic fairytale fates, fervently foremost finally finishing forever.

------------- Part I ---------------