System Information
win7 SP1 and ubuntu 14.04, Radeon HD7770

Blender Version
Broken: Blender 2.70a official and recent own build Hash: 7280410
Worked: ?

Short description of error
Trying to play a Sound strip that has been extended at the start (negative start trim) makes Blender crash

Exact steps for others to reproduce the error

• in the VSE, Add Sound (align start to frame 1)
• in the properties of the strip, enable Cashing
• set Trim Duration (soft) to a negative value for "Start:"
• press Shift-Leftarrow and Blender crashes or set current frame to 0 by RMB-clicking in VSE or timeline and Blender crashes

This not perfectly reproducible: wav, mp3, aif and flac files all crashed but unfortunately not every time even with the same file.
The crash does not occur if the soundfile is packed (might be a coincidence) so you have to create it yourself by loading a sound in the VSE:
Please download this mp3 http://dig.ccmixter.org/files/basematic/33579 or try some of your own files.

blender.crash.txt:

Blender 2.70 (sub 0), Commit date: 2014-04-10 11:49, Hash f93bc76

bpy.context.area.type = 'VIEW_3D' # Property
bpy.data.sounds["basematic_-_Orc_March.mp3"].use_memory_cache = True # Property
bpy.context.scene.sequence_editor.sequences_all["basematic_-_Orc_March.mp3"].frame_offset_start = -1 # Property