You are actually better mixing the two tracks in the video editor. I suspect that you think Track A is reducing in volume when it isn't. It is probably the case that the original volume in Track B was just louder to begin with!
In any case it hardly matters as there are two ways to adjust the volume of any Audio track.
Method 1 - On the Time Line
1. Select the appropriate audio track by Clicking on it.
2. Move your mouse over the line that runs through the middle of the audio track until it becomes a little pointing hand (I am assuming that you are using standard Windows mouse pointers).
3. Left click the line to create an edit point.
4. Repeat 2 and 3 further along the track (30 seconds if I remember correctly in your cae).
5. Now move your pointer along to the first edit point until it is a little pointing hand again. Now simply click and drag (Drag up will increase the volume, Drag down will decrease the volume).
6. Repeat step 5 as many times as you like on as many edit points as you like.
7. Remember though that you will only be able to hear the effect of your edits in the 'Preview Window'
Method 2 - Using the Audio Mixing Panel
This is a much more eloquent method of adjusting the audio in real time.
1. Create a preview range in your project over the 30 seconds or so where you want to adjust the volume.
2. From the 'Window' menu select 'Audio Mixing Panel'
For the next steps I am assuming that your audio clips are on tracks Aa and Ab (these are reflected in the Audio Mixing Panel as 1 and 2 (don't ask why))
3. Click the solo button on Track 1, so that it is coloured red.
4. Click the 'Options' button and select 'Auto Write > Replacement'
5. At the bottom of Track 1 click the 'Auto Write Button'
6. On Track 1 drag the volume up or down to increase or decrease the volume accordingly.
7. Press 'Play the Preview Range' button at the top of the 'Audio Mixing Panel' window.
8. Repeat steps 6 and 7 until you are happy with the levels.
9. When happy click the 'Apply' button at the bottom right of the master track.
10. and lastly click 'Solo' button on track one to turn off the red light.
11. Repeat steps 3 through to 10 for Track 2
There are more sophisticated ways than this to use the 'Audio Mixing Panel' and to experiment with them, simply refer to page 228 of the manual. If you have a Japanese manual then look up the Japanese equivalent of Audio Mixing Panel in the index.
Enjoy
Mixing audio at specific times 
Linear Mode
