Synocommunity's ffmpeg-3.3.3-7 is compiled depends on DSM 5.2 toolchains and can't run correctly on DSM 6.1, so I cloned the spksrc and recompiled ffmpeg 3.3.3 depends on DSM6.1 toolchains (arch is braswell, eg: DS916+). If you need this package, you can download from the link below and try it.
https://mega.nz/#!krwzjSjT!Jey2qLaonjftRRRykknKMfQnXK-4yZ_2ys9a58bXiIs