OK, problem solved. All this was due to the version of Shotcut I was using (17.06.02 in the Debian Stretch release). With this version, when moving the video clip to the timeline, the audio level drops significantly: -13,5dB with the video I was working on (voice audio track) and -6dB with a pure 1kHz tone. This is no more the case in the latest Shotcut release (19.02.28) so I consider that this problem has been addressed and fixed. There is still a difference between the audio level of the video track and the level reported by the peak meter: I recorded a video with a 1kHz tone at 0dB and when played in Shotcut the peak meter shows apprx. +2dB. This is apparently due to oversampling (see this old topic: Shotcut peak meter reads incorrectly - lowers volume while rendering). Thank you all for helping me solve this and sorry for having put up a problem which no longer exists.