ffmpeg -i sample.avi -i subs.srt -y sample.mkv
ffmpeg version N-36274-gbace181, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 30 2011 21:16:24 with gcc 4.5.3
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --mandir=/usr/share/man --enable-shared --cc=x86_64-pc-linux-gnu-gcc --cxx=x86_64-pc-linux-gnu-g++ --ar=x86_64-pc-linux-gnu-ar --optflags='-march=native -O3 -pipe' --extra-cflags='-march=native -O3 -pipe' --extra-cxxflags='-march=native -O3 -pipe' --disable-static --enable-gpl --enable-version3 --enable-postproc --enable-avfilter --disable-stripping --disable-debug --disable-doc --disable-network --disable-vdpau --enable-libmp3lame --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-libaacplus --enable-nonfree --enable-libfaac --enable-nonfree --enable-libcdio --enable-libdc1394 --disable-indev=v4l --disable-indev=oss --disable-indev=jack --enable-x11grab --enable-libv4l2 --disable-outdev=oss --enable-libfreetype --enable-libass --enable-pthreads --enable-libopencore-amrwb --enable-libopencore-amrnb --enable-libgsm --enable-librtmp --enable-libspeex --enable-libv  libavutil      51. 33.100 / 51. 33.100
  libavcodec     53. 49.100 / 53. 49.100
  libavformat    53. 29.100 / 53. 29.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 55.100 /  2. 55.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100

Seems stream 0 codec frame rate differs from container frame rate: 21845.00 (21845/1) -> 23.98 (24000/1001)
Input #0, avi, from 'sample.avi':
  Metadata:
    encoder         : Lavf53.29.100
  Duration: 00:00:10.03, start: 0.000000, bitrate: 492 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46), yuv420p, 624x352 [SAR 1:1 DAR 39:22], 23.98 fps, 23.98 tbr, 23.98 tbn, 21845 tbc
    Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, s16, 128 kb/s
[srt @ 0x20d1ed0] Estimating duration from bitrate, this may be inaccurate
Input #1, srt, from 'subs.srt':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Subtitle: srt
[buffer @ 0x20e6320] w:624 h:352 pixfmt:yuv420p tb:1/1000000 sar:1/1 sws_param:
[libx264 @ 0x20e9510] using SAR=1/1
[libx264 @ 0x20e9510] using cpu capabilities: MMX2 SSE2Fast SSSE3 Cache64
[libx264 @ 0x20e9510] profile High, level 3.0
[libx264 @ 0x20e9510] 264 - core 118 - H.264/MPEG-4 AVC codec - Copyleft 2003-2011 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=3 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=23 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, matroska, to 'sample.mkv':
  Metadata:
    encoder         : Lavf53.29.100
    Stream #0:0: Video: h264, yuv420p, 624x352 [SAR 1:1 DAR 39:22], q=-1--1, 1k tbn, 23.98 tbc
    Stream #0:1: Audio: vorbis, 48000 Hz, stereo, s16, 128 kb/s
    Stream #0:2: Subtitle: ssa
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg4 -> libx264)
  Stream #0:1 -> #0:1 (mp3 -> libvorbis)
  Stream #1:0 -> #0:2 (srt -> ass)
Press [q] to stop, [?] for help
frame=  239 fps= 42 q=-1.0 Lsize=     701kB time=00:00:09.92 bitrate= 578.6kbits/s
video:554kB audio:111kB global headers:5kB muxing overhead 4.753171%
[libx264 @ 0x20e9510] frame I:2     Avg QP:12.66  size:  4951
[libx264 @ 0x20e9510] frame P:218   Avg QP:22.91  size:  2514
[libx264 @ 0x20e9510] frame B:19    Avg QP:24.95  size:   453
[libx264 @ 0x20e9510] consecutive B-frames: 88.3%  2.5%  2.5%  6.7%
[libx264 @ 0x20e9510] mb I  I16..4: 70.7% 26.2%  3.1%
[libx264 @ 0x20e9510] mb P  I16..4:  3.7%  9.0%  0.7%  P16..4: 31.7%  5.5%  3.2%  0.0%  0.0%    skip:46.3%
[libx264 @ 0x20e9510] mb B  I16..4:  0.9%  2.1%  0.0%  B16..8: 20.5%  0.7%  0.1%  direct: 1.0%  skip:74.6%  L0:58.3% L1:39.0% BI: 2.7%
[libx264 @ 0x20e9510] 8x8 transform intra:64.7% inter:88.8%
[libx264 @ 0x20e9510] coded y,uvDC,uvAC intra: 48.8% 61.3% 10.9% inter: 9.4% 9.1% 0.6%
[libx264 @ 0x20e9510] i16 v,h,dc,p: 68% 14% 17%  1%
[libx264 @ 0x20e9510] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 38% 13% 39%  4%  1%  1%  1%  1%  4%
[libx264 @ 0x20e9510] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 67% 18%  8%  1%  1%  1%  1%  1%  1%
[libx264 @ 0x20e9510] i8c dc,h,v,p: 35% 18% 44%  3%
[libx264 @ 0x20e9510] Weighted P-Frames: Y:3.2% UV:2.8%
[libx264 @ 0x20e9510] ref P L0: 70.9%  9.2% 13.6%  6.3%  0.0%
[libx264 @ 0x20e9510] ref B L0: 80.7% 16.5%  2.9%
[libx264 @ 0x20e9510] ref B L1: 87.4% 12.6%
[libx264 @ 0x20e9510] kb/s:454.73