append a string to a modified line

[Mr_Blue]

i'll try to do this in steps:

1- problem: i must rewrite an corrupted m3u file its lines go:

Output:
#EXTINF123#, Title - Song # let's say for clarity sake that the n line

d:\music\title - song.mp3 # this would be the n+1 line

where in fact it should read (caption is important, mind you)

Output:
#EXTINF123#, Title - Song
Title - Song.mp3

first part is to erase the line that include d:\music... and then this gives a line that has to be rewritten just below itself, becoming the nth+1 line

the first part is easy i get a file with plenty of lines that have to modified.

Output:
#EXTINF123#, Title - Song
#EXTINF123#, Title1 - Song1...

so i shift to the main. question

2- tentative solution

import in_place

    with in_place.InPlace('Soothing.m3u') as file:
        for line in file:
            file.write(line)
            file.write(line[12:])

3- (new) problem:

that will not append the ".mp3" string at the end of the second (n+1) line, ie :

Output:
#ext123#, Title - Song
Title - Song

4- (new)tentative solutions

4.1 trying to "add" the string, of course won't do (that would have been too easy ==> no fun)

file.write(line[12:] + ".mp3")

this will add the extension at the beginning of the next line, of course

4.2 "moving" the cursor to the beginning of the n+1 line

with in_place.InPlace('Soothing.m3u') as file:
    for line in file:
        file.write(line)
        file.write(".mp3")
        file.seek(-5,1)
        file.write(line[10:])

this returns an error

Error:
Traceback (most recent call last):
  File "erase.py", line 7, in <module>
    file.seek(-5,1)
AttributeError: 'InPlace' object has no attribute 'seek'

before, you ask: trying to switch to a .txt file won't change anything to the problem. feel free to test it with a text file, of course.

4.3 join i think i have something here. it doesn't work but if you could help me understand why it would be even more interesting, I suspect... of course i wouldn't turn over the answer :)

import in_place

with in_place.InPlace('Soothing.m3u') as file:
    for line in file:
        file.write(line)
        a = line[12:]
        b = ".mp3"      
        c = a.join(b)
        file.write(c)

it just dispatches b all over the place, giving something like:

Output:
#EXTINF:135,Title - Song
.Title - Song
mTitle - Song
pTitle - Song
3#EXTINF:194,Title1 - Song1

any help's welcome.

OS: ubuntu 18.04

python 2.7 but can use python 3.7