The following script is proof-of-concept you might find useful:

<<header::binary-size(10), rest::binary>> = id3tag_bytes
<<"ID3", major::binary-size(1), revision::binary-size(1), flags::binary-size(1), size::integer-size(32)>> = header
<<frame_overview::binary-size(10), frame::binary>> = rest
<<frame_id::binary-size(4), frame_size::unsigned-integer-32, frame_flags::binary-size(2)>> = frame_overview

frame_payload_size = frame_size
frame_payload = <<frame::binary-size(frame_payload_size)>>

{title, encoding} =
  case frame_payload do
    <<1, 0xff, 0xfe, title::binary>> -> {title, {:utf16, :little}}
    <<1, 0xfe, 0xff, title::binary>> -> {title, {:utf16, :big}}
    <<0, title::binary>> -> {title, :iso8859}  
  end

case encoding do
  :iso8859 -> :iconv.convert("iso8859-1", "utf8", title)
  encoding -> :unicode.characters_to_binary(title, encoding, :utf8)
end

Welcome to the Elixir community!

If we look at what the encoding of the expected title is we can see its probably UTF16-LE since its a true subset of the frame payload:

iex> :iconv.convert("utf8", "utf16le", title)
<<76, 0, 97, 0, 32, 0, 81, 0, 117, 0, 234, 0, 116, 0, 101, 0>>

So now we know what we are looking for, we can examine the actual frame content, which appears to be:

iex> <<header::binary-size(10), rest::binary>> = id3tag_bytes
iex> <<"ID3", major::binary-size(1), revision::binary-size(1), flags::binary-size(1), size::binary-size(4)>> = header
iex> <<frame_overview::binary-size(10), frame::binary>> = rest
iex> <<frame_id::binary-size(4), frame_payload_size::unsigned-integer-32, frame_flags::binary-size(2)>> = frame_overview
iex> frame_payload = <<frame::binary-size(frame_payload_size)>>
iex> <<encoding::unsigned-integer-8, payload::binary>> = frame_payload
  
iex> IO.inspect(frame_payload_size, label: "Frame Payload Size")
Frame Payload Size: 19
iex> IO.inspect encoding, label: "Payload encoding (0 == ISO8859, 1 == UTF16)"
Payload encoding (0 == ISO8859, 1 == UTF16): 1
iex> IO.inspect payload, label: "Frame payload"
Frame payload: <<255, 254, 76, 0, 97, 0, 32, 0, 81, 0, 117, 0, 234, 0, 116, 0, 101, 0>>

Observations of the frame payload

1. <<255, 254>> is the BOM indicating UTF16 little endian.
2. The rest of the data is the actual string which we can decode with the following. Note that we don’t include the BOM, but we do use it to determine the endianness for conversion:

iex> title = <<76, 0, 97, 0, 32, 0, 81, 0, 117, 0, 234, 0, 116, 0, 101, 0>>
iex> :unicode.characters_to_binary(title, {:utf16, :little}, :utf8)
"La Quête"

Note that :unicode.characters_to_binary/3 does not detect endianness from a BOM, so the BOM needs to be excluded. :iconv.convert/3 has the same consideration.

Possible next steps

1. For ISO-8859-1 conversion you may need to include :iconv as a dependency.

IO.read and File.read return different types. chardata() is not the same as binary()

IO.read/2
@spec read(device(), :eof | :line | non_neg_integer()) :: chardata() | nodata()

File.read/1
@spec read(Path.t()) :: {:ok, binary()} | {:error, posix()}

In your case, I think I would just be doing:

case File.read(path) do
  {:ok, binary} -> extract_tags(binary)
  error -> error
end

Also note that I edited the case clause for :iso8859 to correctly binary pattern match.

doesn’t this solve it?