diff --git a/aliases b/aliases
index b49a49a..51e6781 100644
--- a/aliases
+++ b/aliases
@@ -368,7 +368,9 @@ dl_youtube_vid() {
     fi
     if [[ $format == "" ]]; then
         printf "${BOLD}No format given; using best available.${NORMAL}\n"
-        format="best"
+        # Download best mp4 video and best m4a audio, then merge.
+        format="bv*[ext=mp4]+ba[ext=m4a]"
+        opts+=" --merge-output-format mp4"
     fi
 
     opts+=" --write-sub --sub-lang en --embed-subs"
@@ -411,7 +413,9 @@ dl_youtube_vid_and_hflip() {
     fi
     if [[ $format == "" ]]; then
         printf "${BOLD}No format given; using best available.${NORMAL}\n"
-        format="best"
+        # Download best mp4 video and best m4a audio, then merge.
+        format="bv*[ext=mp4]+ba[ext=m4a]"
+        opts+=" --merge-output-format mp4"
     fi
 
     opts+=" --write-sub --sub-lang en --embed-subs"
@@ -453,7 +457,9 @@ dl_youtube_playlist() {
     fi
     if [[ $format == "" ]]; then
         printf "${BOLD}No format given; using best available.${NORMAL}\n"
-        format="best"
+        # Download best mp4 video and best m4a audio, then merge.
+        format="bv*[ext=mp4]+ba[ext=m4a]"
+        opts+=" --merge-output-format mp4"
     fi
 
     opts+=" --write-sub --sub-lang en --embed-subs"
@@ -540,6 +546,7 @@ dl_twitch_vid() {
     if [[ $format == "" ]]; then
         printf "${BOLD}No format given; using best available.${NORMAL}\n"
         format="best"
+        #opts+=" --merge-output-format mp4"
     fi
 
     if [[ $make_folder == "1" ]]; then