find . -type f -name '*.[Mm][Pp]3' -exec id3 -Rl {} + -o \
  -name '*.[Ff][Ll][Aa][Cc]' -exec metaflac --show-md5sum  \
    --with-filename --export-tags-to=- {} + |
  awk 'BEGIN {
    mp3n        = split("album artist title track",       mp3_tags)
    flacn       = split("album artist title tracknumber", flac_tags)
    ignorepatt  = "^ *(unknown|track *[0-9]*)* *$"    
    }
  $1 ~ /\.[Ff][Ll][Aa][Cc]$/ {
    if (fn) {
      for (i = 1; i <= flacn; i++) {
        if (tolower(tags[flac_tags[i]]) ~ ignorepatt) {
          invalid_tags[flac_tags[i]] = tags[flac_tags[i]]
          f || f++
          }
        }
      if (f) {
       print RS, fn, "has missing/invalid tags:"
        for (t in invalid_tags)
          printf "%s -> %s\n", t, invalid_tags[t]
        }      
      }    
    split(x, tags); split(x, invalid_tags)
    fn = $1; f = x
    }
  /^Filename/ {
    if (fn) {
      for (i = 1; i <= mp3n; i++) {
        if (tolower(tags[mp3_tags[i]]) ~ ignorepatt) {
          invalid_tags[mp3_tags[i]] = tags[mp3_tags[i]]
          f || f++
          }
        }
      if (f) {
       print RS, fn, "has missing/invalid tags:"
        for (t in invalid_tags)
          printf "%s -> %s\n", t, invalid_tags[t]
        }      
      }    
    split(x, tags); split(x, invalid_tags)
    fn = $2; f = x
    }
 { 
    if (split($0, tmp, "=") == 2) {
      $1 = tmp[1]; $2 = tmp[2]
      }
    tags[tolower($1)] = $2
    }
  END {
      if (fn ~ /\.[Ff][Ll][Aa][Cc]$/) {
        for (i = 1; i <= flacn; i++) {
          if (tolower(tags[flac_tags[i]]) ~ ignorepatt) {
            invalid_tags[flac_tags[i]] = tags[flac_tags[i]]
            f || f++
            }
          }
        if (f) {
          print RS, fn, "has missing/invalid tags:"
          for (t in invalid_tags)
            printf "%s -> %s\n", t, invalid_tags[t]
            }      
          }    
    else       
      for (i = 1; i <= mp3n; i++) {
        if (tolower(tags[mp3_tags[i]]) ~ ignorepatt) {
          invalid_tags[mp3_tags[i]] = tags[mp3_tags[i]]
          f || f++
          }
      if (f) {
        print RS, fn, "has missing/invalid tags:"
          for (t in invalid_tags)
          printf "%s -> %s\n", t, invalid_tags[t]
          }            
        }  
      }' FS=: