Add audio length var to env sample

Added MAX_AUDIO_LENGTH var and documented in .env.production.sample

.env.production.sample

@@ -175,6 +175,9 @@ STREAMING_CLUSTER_NUM=1
 # MAX_IMAGE_SIZE=8388608
 # MAX_VIDEO_SIZE=41943040
 
+# Maximum length of audio uploads in seconds
+# MAX_AUDIO_LENGTH=60
+
 # LDAP authentication (optional)
 # LDAP_ENABLED=true
 # LDAP_HOST=localhost