How to remove voices from a score

Regarding voices...
If you press Ctrl+A, it will select everything in your score and you will see that all the notes are colored blue, and so are in voice 1. There is no need to delete any voices.
See:
https://musescore.org/en/handbook/3/voices#How_voices_are_displayed

Regarding the extra rests and + signs...
A small - or + sign appears above the measure when its duration is different than indicated by the time signature.
See:
https://musescore.org/en/handbook/3/measure-operations#duration

It all depends on a variety of factors, viz.

1. Is the sound clip a Dolby Surround clip or stereo?
2. What is the bitrate of the sound you have?
3. If the sound clip is Dolby Surround, is the BGM strictly off-center, or are there vocals?
4. Do I have the specific song/sound bite I want to remove from the audio?

If the answer to 3 is that the BGM is strictly off-center, you can simply extract the channels from the 5.1 or 7.1 audio as mono tracks, and then mix down both the center channel and the source audio to stereo. Then you can line both tracks up in Audacity or Adobe Audition, and simply remove the portion from the source audio track (which is now in stereo) by using a cut tool, that has the BGM, and similarly delete all portions of the center channel, except those you want to keep. If there are vocals, and/or the audio is already in stereo, things become a bit more complicated. If the sound bite is 5.1, with center-panned vocals, you will need to mix down the track to mono. Then, if you don’t have a copy (or OST version) of the offending audio you want to remove, you can try to reduce the music as much as possible using the Acapella preset of the center channel extractor in Adobe Audition (you can modify the preset by specifying the vocal spectrum, viz. female, male, or full spectrum. It also helps if there is a part of the audio with just the music. You can use the use the Sound Remover effect in Adobe Audition CC to remove this residual sound. Please note that is a very imprecise process, and results may vary. If you have the OST version of the sound you want to remove, and it’s bitrate is close to that of your audio, things get a little easier, albeit time-consuming. If the music is not auto-ducked on the dialogue (that is to say, if the volume or amplitude of the music does not depend on the dialogue) then you can simply match the volume of the OST version of your track to the audio clip you’re editing, and phase-cancel out the music from the audio clip, or use the Sound Remover effect. If, on the other hand, the music is auto-ducked, you will need to play it by ear, and match the amplitudes of different parts of the OST sound bite to their counterparts in the audio clip you’re editing, depending on the length of the audio clip, and then using the Sound Remover effect. Any residual sound can be cleaned up using the Sound Remover and Noise Reduction effects. Creating your own personalised presets in the Sound Remover effect would help too. You might also want to view videos regarding vocal extraction