Across:
1 Provides feedback (SOUNDING BOARD) to Express, in a manner of speaking (express/utter = SOUND-), on popular (-IN-) government (-G) quango (BOARD) (SOUNDING BOARD),
10 No ads in El Salvador (no ‘ads’ in ‘El Salvador’ = ellvaor = OVERALL) by and large (OVERALL),
11 A (A-) part of an unfinished (l) song (‘vocal’ without ‘l’ = -VOCA-) by engineer (-DO) (AVOCADO) – the green type that’s stoned (AVOCADO),
12 Hawk’s living accommodation (NEST) in the Sunshine State (in the ‘sunshiNE STate’ = NEST),
13 Fit (SOUND) to lead all sides today (SOUND on all sides of today’s grid),
15 Shock (STUN) of truants missing Art (‘truants’ missing ‘art’ = unts = STUN),
17 Auden left unsteady (‘Auden’ left ‘unsteady’ = sty = STY), messy place (STY),
19 Prisoner (prisoner of war = POW-) expresses admiration (-WOW) (POWWOW) for US Congress? (POWWOW),
21 Rome by (Rome by = EMBYRO) train? (= anagram indicator) That’s in the early stage of development! (EMBYRO),
22 Rings a (rings a = SIGN-RA) criminal (= anagram indicator) about old (-O-) (SIGNORA) woman living in Sicily (SIGNORA),
23 Garda (cop) loses the head (c) (‘cop’ without ‘c’ = OP-) with local supplier of The Ticket (ticket tout = -T OUT) (OPT OUT) in decline (decline/refuse = OPT OUT),
25 Mysterious (ARCANE) old sailor (tar) dismisses top (t) (‘tar’ without ‘t’ = AR-) staff (-CANE) (ARCANE),
27 Some definition (some ‘deFINition’ = FIN) of a codpiece? (piece of cod = FIN),
29 Figure (FORM) tries to get away from Firestorm (‘tries’ to get away from ‘firestorm’ = from = FORM),
30 Novel idea (plot = P-LOT) about one (-I-) (PILOT) employed by the Air Corps (PILOT),
31 Score (ETCH) produced by lead character (lead ‘Character’ = -C-) in The (the = ET-H) Rock (= anagram indicator) (ETCH),
34 Goads (NEEDLES) the French (French ‘the’ = -LE-) in dense (dense = NEED-S) novel (= anagram indicator) (NEEDLES),
35 One of eight mouthy types (INCISOR tooth) of Victorians rejected VAT (“Victorians” rejected “VAT” = icorins = INCISOR),
36 You won’t find any holes in this (SOUND ARGUMENT) – that’s why it holds water presumably (SOUND ARGUMENT).
Down:
2 Supervised the staff (OVERSAW) in the chippy’s place presumably (chippy’s/carpenter’s place = over saw = OVERSAW),
3 Orderly (NEAT) produces sample of medicine attentively (sample of ‘mediciNE ATtentively’ = NEAT),
4 One (I-) key (music key = -G-) for the gents (-LOOS) (IGLOOS) in blockhouses? (ice blocks for IGLOOS),
5 Like some old newsreels (GRAINY) of Grey (‘grey’ = ‘gray’ = GRA-Y) on the telephone (= homophone indicator) entertaining Ulster (Ulster= Northern Ireland/ NI = -IN-) Rising (= reversal indicator) (GRAINY),
6 Instrument (OBOE) picked up (= reversal indicator) in archaeobotany (picked up in ‘archaEOBOtany’ = eobo = OBOE),
7 Sob from Serbo-Croat (‘sob’ from ‘serbo-croat’ = ercroat = REACTOR) working in nuclear installation (REACTOR),
8 Is this why we’ve heard nothing from The Assembly (SOUNDPROOFING), barring the racket they’re making in The House? (barring/preventing noise = SOUNDPROOFING),
9 Give warning to those out driving (SOUND ONE’S HORN) you’re likely to hear this (SOUND) individual’s (ones = ONE’S) from The Cape (Cape HORN) (SOUND ONE’S HORN),
14 Tool (UTENSIL) for moving (= anagram indicator) sun tile (sun tile = UTENSIL),
16 Shift (= anagram indicator) bonus (bonus = BOSUN) for one of the crew (ship’s officer = BOSUN),
18 Fashionably dressed (SMART) dame from Amsterdam (‘dame’ from ‘Amsterdam’ = amstr = SMART),
20 Waiter hasn’t got ear (‘waiter’ hasn’t got ‘ear’ = wit = WIT) of comedian (WIT),
21 Raised (= reversal indicator) in the banana republic (in the ‘bananA REpublic’ = are = ERA) from a young age? (ERA),
24 In retrospect (= reversal indicator), old (-O-) railway heading (“Railway” heading =-R-) out of train station (depot = TO-PED-) (TORPEDO) is explosive for those in the shipping game (TORPEDO),
26 American (A-) trains (trains = -RTISAN) run (= anagram indicator) (ARTISAN) by one of the skilful manual workers (ARTISAN),
27 Included in selfie staggering (included in ‘selFIE STAggering’ = FIESTA) out of Spanish celebration (FIESTA),
28 Recording (NOTING) away (not in = NOTIN-) game from the start (‘Game’ from the start = -G) (NOTING),
32 Dynasty (CLAN) from the eternal city (from the ‘eterNAL City’ = nalc = CLAN) going all the way back (= reversal indicator),
33 Scotsman loses tons (‘Scotsman’ loses ‘tons’ = scma = SCAM) in fraudulent scheme (SCAM).