2nd try at committing to trunk (instead of particles branch) :)

Fixes for bugs in #7833
- Hair softbody cache was reset on rendering.
- When Alt-a playback is synced to audio frames are skipped. This caused dynamics caching to go crazy. Now the first loop of playback goes through all frames if there are non cached dynamic systems.

Invalid point caches weren't cleared when new systems were created. This caused messed up shapes when softbody was added.

source/blender/blenkernel/BKE_pointcache.h

@@ -44,5 +44,6 @@
 int		BKE_ptcache_id_filename(struct ID *id, char *filename, int cfra, int stack_index, short do_path, short do_ext);
 FILE *	BKE_ptcache_id_fopen(struct ID *id, char mode, int cfra, int stack_index);
 void	BKE_ptcache_id_clear(struct ID *id, char mode, int cfra, int stack_index);
+int		BKE_ptcache_id_exist(struct ID *id, int cfra, int stack_index);
 
 #endif

source/blender/blenkernel/intern/particle_system.c

@@ -4271,7 +4271,7 @@ void psys_to_softbody(Object *ob, ParticleSystem *psys, int force_recalc)
 
 	if((psys->softflag&OB_SB_ENABLE)==0) return;
 
-	if((ob->recalc&OB_RECALC_TIME)==0)
+	if(ob->recalc && (ob->recalc&OB_RECALC_TIME)==0)
 		psys->softflag|=OB_SB_REDO;
 
 	/* let's replace the object's own softbody with the particle softbody */

source/blender/blenkernel/intern/pointcache.c

@@ -183,3 +183,11 @@ void BKE_ptcache_id_clear(struct ID *id, char mode, int cfra, int stack_index)
 	return;
 }
 
+int BKE_ptcache_id_exist(struct ID *id, int cfra, int stack_index)
+{
+	char filename[(FILE_MAXDIR+FILE_MAXFILE)*2];
+	
+	BKE_ptcache_id_filename(id, filename, cfra, stack_index, 1, 1);
+
+	return BLI_exists(filename);
+}

source/blender/src/buttons_object.c

@@ -3248,6 +3248,7 @@ static void object_softbodies__enable(void *ob_v, void *arg2)
 		if (!ob->soft) {
 			ob->soft= sbNew();
 			ob->softflag |= OB_SB_GOAL|OB_SB_EDGES;
+			softbody_clear_cache(ob, CFRA);
 		}
 	}
 	/* needed so that initial state is cached correctly */
@@ -3281,6 +3282,7 @@ static void object_softbodies__enable_psys(void *ob_v, void *psys_v)
 			psys->soft= sbNew();
 			psys->softflag |= OB_SB_GOAL|OB_SB_EDGES;
 			psys->soft->particles=psys;
+			clear_particles_from_cache(ob, psys, CFRA);
 		}
 		psys->softflag |= OB_SB_ENABLE;
 	}

source/blender/src/drawview.c

@@ -70,6 +70,8 @@
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
 #include "DNA_meta_types.h"
+#include "DNA_modifier_types.h"
+#include "DNA_object_force.h"
 #include "DNA_object_types.h"
 #include "DNA_particle_types.h"
 #include "DNA_screen_types.h"
@@ -101,8 +103,10 @@
 #include "BKE_key.h"
 #include "BKE_main.h"
 #include "BKE_mesh.h"
+#include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_pointcache.h"
 #include "BKE_scene.h"
 #include "BKE_texture.h"
 #include "BKE_utildefines.h"
@@ -3292,10 +3296,50 @@ static void inner_play_prefetch_shutdown(int mode)
 	seq_stop_threads();
 }
 
+static int cached_dynamics(int sfra, int efra)
+{
+	Base *base = G.scene->base.first;
+	Object *ob;
+	ModifierData *md;
+	ParticleSystem *psys;
+	int i, stack_index, cached=1;
+
+	while(base && cached) {
+		ob = base->object;
+		if(ob->softflag & OB_SB_ENABLE && ob->soft) {
+			for(i=0, md=ob->modifiers.first; md; i++, md=md->next) {
+				if(md->type == eModifierType_Softbody) {
+					stack_index = i;
+					break;
+				}
+			}
+			for(i=sfra; i<=efra && cached; i++)
+				cached &= BKE_ptcache_id_exist(&ob->id,i,stack_index);
+		}
+
+		for(psys=ob->particlesystem.first; psys; psys=psys->next) {
+			stack_index = modifiers_indexInObject(ob,(ModifierData*)psys_get_modifier(ob,psys));
+			if(psys->part->type==PART_HAIR) {
+				if(psys->softflag & OB_SB_ENABLE && psys->soft);
+				else
+					stack_index = -1;
+			}
+
+			if(stack_index >= 0)
+				for(i=sfra; i<=efra && cached; i++)
+					cached &= BKE_ptcache_id_exist(&ob->id,i,stack_index);
+		}
+		
+		base = base->next;
+	}
+
+	return cached;
+}
 void inner_play_anim_loop(int init, int mode)
 {
 	ScrArea *sa;
 	static int last_cfra = -1;
+	static int cached = 0;
 
 	/* init */
 	if(init) {
@@ -3304,7 +3348,7 @@ void inner_play_anim_loop(int init, int mode)
 		tottime= 0.0;
 		curmode= mode;
 		last_cfra = -1;
-
+		cached = cached_dynamics(PSFRA,PEFRA);
 		return;
 	}
 
@@ -3380,8 +3424,10 @@ void inner_play_anim_loop(int init, int mode)
 		CFRA = PSFRA;
 		audiostream_stop();
 		audiostream_start( CFRA );
+		cached = cached_dynamics(PSFRA,PEFRA);
 	} else {
-		if (U.mixbufsize 
+		if (cached
+			&& U.mixbufsize 
 		    && (G.scene->audio.flag & AUDIO_SYNC)) {
 			CFRA = audiostream_pos();
 		} else {