This is consistently reproducible on my system (current Tumbleweed,
Lneovo T450 with Broadwell-U Integrated Graphics, see below).

When gstreamer-plugins-vaapi is installed, totem plays the sound, but
no video output for MP4s, FLVs,...  Once gstreamer/VAAPI is removed,
everything is back to normal.

This feels similar to bug #951150 from a year ago.

# hwinfo --gfx
08: PCI 02.0: 0300 VGA compatible controller (VGA)              
  [Created at pci.374]
  Unique ID: _Znp.FN2jBW+YDR6
  SysFS ID: /devices/pci0000:00/0000:00:02.0
  SysFS BusID: 0000:00:02.0
  Hardware Class: graphics card
  Model: "Intel Broadwell-U Integrated Graphics"
  Vendor: pci 0x8086 "Intel Corporation"
  Device: pci 0x1616 "Broadwell-U Integrated Graphics"
  SubVendor: pci 0x17aa "Lenovo"
  SubDevice: pci 0x5034 
  Revision: 0x09
  Driver: "i915"
  Driver Modules: "drm"
  Memory Range: 0xe0000000-0xe0ffffff (rw,non-prefetchable)
  Memory Range: 0xd0000000-0xdfffffff (ro,non-prefetchable)
  I/O Ports: 0x3000-0x303f (rw)
  Memory Range: 0x000c0000-0x000dffff (rw,non-prefetchable,disabled)
  IRQ: 43 (1042676 events)
  I/O Ports: 0x3c0-0x3df (rw)
  Module Alias: "pci:v00008086d00001616sv000017AAsd00005034bc03sc00i00"
  Driver Info #0:
    Driver Status: i915 is active
    Driver Activation Cmd: "modprobe i915"
  Config Status: cfg=new, avail=yes, need=no, active=unknown