PHP, 3311 2988 2916 2845 2759 2671 26672667 2509 2484 bytes, passing 301/400
<?define(J,[ieh,ah,aa,Ah,eh,ee,ie,ih,oh,oo,Oh,uh,Uh,au,eu,Au,ei,ai,ey,ay,a,e,i,o,u,A,O,U,y]);$b=$e=[];function v($a){return in_array($a,J);}function n($a){return!v($a);}function pronounce($w){global$b,$e;$w=strtr(strtolower(strtr($w,[ß=>5,Ä=>4,ä=>4,Ö=>0,ö=>0,Ü=>6,ü=>6])),[5=>S,4=>A,0=>O,6=>U]);$W=[];$L=$w;while($L)foreach(array_merge([tzsch,dsch,tsch,zsch,sch,chs,ch,ck,dt,ng,nk,pf,ph,qu,ss,th,tz,b,c,d,f,g,h,j,k,l,m,n,p,r,s,S,t,v,w,x,z],J)as$c){$l=strlen($c);if(substr($L,0,$l)==$c){$W[]=$c;$L=substr($L,$l);break;}}$s=[];$l=count($W);for($o=$t=$i=0;$i<$l;$i++){$c=$W[$i];$p=$i?$W[$i-1]:0;$n=$i==$l-1?0:$W[$i+1];if(!$n||!(n($c)&&$c==$n&&n($W[$i+2])))$s[$o][]=$c;if($p&&((!$n&&v($c)&&n($p))||(n($n)&&v($W[$i+2]))))$s[++$o]=[];}$s[count($s)-1]||array_pop($s);foreach($s as$z){$b[]=$t;$t+=count($z);$e[]=$t;}$o=[sie=>ziQ,du=>duQ,'die'=>diQ,der=>deQAB,zu=>tsuQ,wir=>viQAB,mir=>miQAB,den=>deQn,dass=>das,er=>eQAB,so=>zoQ,war=>vaQAB,für=>fyQAB,ja=>jaQ,wie=>viQ,dir=>diQAB,nur=>nuQAB,dem=>deQm,ihn=>iQn,auch=>aUBx,ihr=>iQAB,da=>daQ,schon=>ʃoQn,wenn=>vEn,mal=>maQl,gut=>guQt,nach=>naQx,will=>vIl,muss=>mUs,hab=>haQp,vor=>foQAB,ihm=>miQm,tun=>tuQn][$w]?:'';if($o)goto x;$P=$B=$S=$V=$Z=0;count($s)>1&&$o=[ver=>fEAB,da=>da,ge=>gC][join($s[0])];$o&&$P=1&&array_shift($s);(($P||count($s)==2)&&$o.=W)||(count($s)>2&&$d=1&&$h=(int)count($s)/2);$w=array_merge(...$s);foreach($w as$l){if(0<$S--)continue;$n=$w[$B+1];$p=$w[$B-1];$f='';$Z+=in_array($B,$b);$f=$l==d&&!$n?t:($l==b&&(!$n||$n==t)?p:($l==g&&(!$n||$n==t)?((!$n&&$p==i)?K:k):($l==c?(($n==A||$n==e||$n==i)?ts:k):($l==ch?(($p==a||$p==o||$p==u)?x:K):($l==chs||$l==x?ks:($l==ck?k:($l==dsch?dZ:($l==dt||$l==th?t:($l==ph||$l==v?f:(!$f&&$l==g?g:($l==h?(v($n)?h:''):($l==ng?N:($l==nk?Nk:($l==qu?kv:($l==r?((!v($n)&&!$n==t)?AB:R):($l==sch?S:($l==ss||$l==S?s:($l==tsch||$l==tzsch||$l==zsch?tS:($l==tz||$l==z?ts:($l==w?v:($l==s?(!$p&&($n==p||$n==t))?S:(v($n)?z:s):$f)))))))))))))))))))));$U=0;if(v($l)){if(strlen($l)>1)($f=[au=>aUB,eu=>cYB,ei=>aIB][$l])||$U=1;else{if(n($n)&&((!$w[$B+2]&&$n!=n)||v($w[$B+2])))$U=1;if($l==e){$U=!$n?:$U;if(!$w[$B+2]){if($n==r)($f=A)&&$U=!$S=1;if($n==m||$n==l)$f=C;}}elseif($n==ch)$U=0;if(in_array($B,$e))$U=0;}$f=($U||!$Z)&&!$f?($l[0]==O?D:($l[0]==U?y:($l[0]==A?E:$l[0]))).Q:(!$f?($l==e?((!$n||(!$w[$B+2]&&($n==n||$n==s)))?C:E):($l==A?E:($l==i?I:($l==o?c:($l==u?U:(($l==U||$l==y)?Y:$l)))))):$f);$V++;}$f=$f?:$l;if($d&&$Z==$h)$f.=W;$o.=$f;$B++;}if(strpos($o,W)===false&&$V>1)$o=W.$o;for(;$j++<strlen($o);)if($o[$j]==$o[$j+1])$o=substr($o,0,$j).substr($o,$j+1);x:return strtr($o,[S=>ʃ,Z=>ʒ,K=>ç,g=>ɡ,N=>ŋ,Q=>'ː',W=>ˈ,B=>'̯',R=>ʁ,A=>ɐ,E=>ɛ,O=>œ,I=>ɪ,Y=>ʏ,U=>ʊ,c=>ɔ,C=>ə,D=>ø]);}
<?$f=fopen(__FILE__,r);fseek($f,__COMPILER_HALT_OFFSET__);eval(strtr(stream_get_contents($f),[F=>'=>',M=>'==','&'=>'&&',H=>'function ',A_=>'array',G=>'if',4=>'for','|'=>'||','~'=>'))','%'=>str,7=>'=$',8=>'[]',9=>'!$','@'=>'count(','#'=>';$',5=>'return ',3=>':(']));__halt_compiler();define(J,[ieh,ah,aa,Ah,eh,ee,ie,ih,oh,oo,Oh,uh,Uh,au,eu,Au,ei,ai,ey,ay,a,e,i,o,u,A,O,U,y])#b7e=8;Hv($a){5in_A_($a,J);}Hn($a){5!v($a);}Hpronounce($w){global$b,$e#w=%tr(%tolower(%tr($w,[ßF1,ÄF2,äF2,ÖF0,öF0,ÜF6,üF6]~,[1FS,2FA,0FO,6FU])#W=8#L7w;while($L)4each(A__merge([tzsch,dsch,tsch,zsch,sch,chs,ch,ck,dt,ng,nk,pf,ph,qu,ss,th,tz,b,c,d,f,g,h,j,k,l,m,n,p,r,s,S,t,v,w,x,z],J)as$c){$l=%len($c);G(sub%($L,0,$l)M$c){$W87c#L=sub%($L,$l);break;}}$s=8#l=@$W);4($o7t7i=0#i<$l#i++){$c7W[$i]#p7i?$W[$i-1]:0#n7iM$l-1?0:$W[$i+1];G(9n|!(n($c)&$cM$n&n($W[$i+2]~)$s[$o]87c;G($p&((9n&v($c)&n($p~|(n($n)&v($W[$i+2]~~$s[++$o]=8;}$s[@$s)-1]|A__pop($s);4each($s as$z){$b87t#t+=@$z)#e87t;}$o=[sieFziQ,duFduQ,'die'FdiQ,derFdeQT,zuFtsuQ,wirFviQT,mirFmiQT,denFdeQn,dassFdas,erFeQT,soFzoQ,warFvaQT,fürFfyQT,jaFjaQ,wieFviQ,dirFdiQT,nurFnuQT,demFdeQm,ihnFiQn,auchFaUBx,ihrFiQT,daFdaQ,schonFʃoQn,wennFvEn,malFmaQl,gutFguQt,nachFnaQx,willFvIl,mussFmUs,habFhaQp,vorFfoQT,ihmFmiQm,tunFtuQn][$w]?:'';G($o)goto x#P7B7S7V7Z=0;@$s)>1&$o=[verFfET,daFda,geFgC][join($s[0])]#o&$P=1&A__shGt($s);(($P|@$s)M2)&$o.=W)|(@$s)>2&$d=1&$h=(int)@$s)/2)#w=A__merge(...$s);4each($w as$l){G(0<$S--)continue#n7w[$B+1]#p7w[$B-1]#f=''#Z+=in_A_($B,$b)#f7lMd&9n?t3$lMb&(9n|$nMt)?p3$lMg&(9n|$nMt)?((9n&$pMi)?K:k)3$lMc?(($nMA|$nMe|$nMi)?ts:k)3$lMch?(($pMa|$pMo|$pMu)?x:K)3$lMchs|$lMx?ks3$lMck?k3$lMdsch?dZ3$lMdt|$lMth?t3$lMph|$lMv?f39f&$lMg?g3$lMh?(v($n)?h:'')3$lMng?N3$lMnk?Nk3$lMqu?kv3$lMr?((!v($n)&9nMt)?T:R)3$lMsch?S3$lMss|$lMS?s3$lMtsch|$lMtzsch|$lMzsch?tS3$lMtz|$lMz?ts3$lMw?v3$lMs?(9p&($nMp|$nMt~?S3v($n)?z:s):$f~~~~~~~~~~)#U=0;G(v($l~{G(%len($l)>1)($f=[auFaUB,euFcYB,eiFaIB][$l])|$U=1;else{G(n($n)&((9w[$B+2]&$n!=n)|v($w[$B+2]~)$U=1;G($lMe){$U=9n?:$U;G(9w[$B+2]){G($nMr)($f=A)&$U=9S=1;G($nMm|$nMl)$f=C;}}elseG($nMch)$U=0;G(in_A_($B,$e~$U=0;}$f=($U|9Z)&9f?($l[0]MO?D3$l[0]MU?y3$l[0]MA?E:$l[0]~).Q39f?($lMe?((9n|(9w[$B+2]&($nMn|$nMs~)?C:E)3$lMA?E3$lMi?I3$lMo?c3$lMu?U3($lMU|$lMy)?Y:$l~~~:$f)#V++;}$f7f?:$l;G($d&$ZM$h)$f.=W#o.7f#B++;}G(%pos($o,W)M=false&$V>1)$o=W.$o;4(#j++<%len($o);)G($o[$j]M$o[$j+1])$o=sub%($o,0,$j).sub%($o,$j+1);x:5%tr($o,[SFʃ,ZFʒ,KFç,gFɡ,NFŋ,QF'ː',WFˈ,TFɐ̯,BF'̯',RFʁ,AFɐ,EFɛ,OFœ,IFɪ,YFʏ,UFʊ,cFɔ,CFə,DFø]);}
Powered by the tears of ternary statements.
EDIT 7: Squeezed off ~170 bytes by writing a preprocessor into the program. As a consequence, the actual program (everything after __halt_compiler();) is kinda hard to read. If you want the unprocessed program, switch eval with print in the third statement.